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OF 


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Co *GBESS 


SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 





This document contains information affecting the national defense of the United 
States within the meaning of the Espionage Act, 50 U.S.C., 31 and 32, as 
amended. Its transmission or the revelation of its contents in any manner to 
an unauthorized person is prohibited by law. 


This volume is "classified in accordance with security regulations of 


the War and Navy Departments because certain chapters contain material 
which was SECRET at the date of printing. Other chapters may have had 
a lower classification or none. The reader is advised to consult the War and 
Navy agencies listed on the reverse of this page for the current classification 
of any material. 


DECU^ tFlFT> 

By 2*3* 

c£P 1 \96Q 


of 


„ meW o 2 August 1W 

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anuscript and illustrations for this volume were prepared for 
publication by the Summary Reports Group of the Columbia 
University Division of War Research under contract OEMsr-1131 
with the Office of Scientific Research and Development. This vol- 
ume was printed and bound by the Columbia University Press. 


Distribution of the Summary Technical Report of NDRC has been 
made by the War and Navy Departments. Inquiries concerning the 
availability and distribution of the Summary Technical Report 
volumes and microfilmed and other reference material should be 
addressed to the War Department Library, Room 1A-522, The 
Pentagon, Washington 25, D. C., or to the Office of Naval Re- 
search, Navy Department, Attention: Reports and Documents 
Section, Washington 25, D. C. 


Copy No. 

238 


This volume, like the seventy others of the Summary Technical 
Report of NDRC, has been written, edited, and printed under 
great pressure. Inevitably there are errors which have slipped past 
Division readers and proofreaders. There may be errors of fact not 
known at time of printing. The author has not been able to follow 
through his writing to the final page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports and sent 
to recipients of the volume. Your help will make this book more 
useful to other readers and will be of great value in preparing any 
revisions. 


SUMMARY TECHNICAL REPORT OF DIVISION 6, NDRC 


VOLUME 22 


ACOUSTIC TORPEDOES 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 6 
JOHN T. TATE, CHIEF 


WASHINGTON, D. C 1946 



NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative 1 

Frank B. Jewett Navy Representative 2 

Karl T. Compton Commissioner of Patents 3 

Irvin Stewart, Executive Secretary 


1 Army representatives in order of service: 


2 Navy representatives in order of service: 


Maj. Gen. 
Maj. Gen. 
Maj. Gen. 
Brig. Gen. 


G. V. Strong 
R. C. Moore 
C. C. Williams 
W. A. Wood, Jr. 
Col. E. 


Col. L. A. Denson 
Col. P. R. Faymonville 
Brig. Gen. E. A. Regnier 
Col. M. M. Irvine 
A. Routheau 


Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 

Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 

Commodore H. A. Schade 
3 Commissioners of Patents in order of service: 

Conway P. Coe Casper W. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities of 
warfare, together with contract facilities for carrying out 
these projects and programs, and (2) to administer the tech- 
nical and scientific work of the contracts. More specifically, 
NDRC functioned by initiating research projects on re- 
quests from the Army or the Navy, or on requests from an 
allied government transmitted through the Liaison Office 
of OSRD, or on its own considered initiative as a result of 
the experience of its members. Proposals prepared by the 
Division, Panel, or Committee for research contracts for 
performance of the work involved in such projects were 
first reviewed by NDRC, and if approved, recommended to 
the Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of the 
contract, including such matters as materials, clearances, 
vouchers, patents, priorities, legal matters, and administra- 
tion of patent matters were handled by the Executive Sec- 
retary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A — Armor and Ordnance 

Division B — Bombs, Fuels, Gases, & Chemical Problems 
Division C — Communication and Transportation 
Division D — Detection, Controls, and Instruments 
Division E — Patents arid Inventions 


In a reorganization in the fall of 1942, twenty-three ad- 
ministrative divisions, panels, or committees were created, 
each with a chief selected on the basis of his outstanding 
work in the particular field. The NDRC members then be- 
came a reviewing and advisory group to the Director of 
OSRD. The final organization was as follows: 

Division 1 — Ballistic Research 

Division 2 — Effects of Impact and Explosion 

Division 3 — Rocket Ordnance 

Division 4 — Ordnance Accessories 

Division 5 — New Missiles 

Division 6 — Sub-Surface Warfare 

Division 7 — Fire Control 

Division 8 — Explosives 

Division 9 — Chemistry 

Division 10 — Absorbents and Aerosols 

Division 11 — Chemical Engineering 

Division 12 — Transportation 

Division 13 — Electrical Communication 

Division 14 — Radar 

Division 15 — Radio Coordination 

Division 16 — Optics and Camouflage 

Division 17 — Physics 

Division 18 — War Metallurgy 

Division 19 — Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 


iv 


Library of Congress 




NDRC FOREWORD 


A s events of the years preceding 1940 revealed 
more and more clearly the seriousness of the 
world situation, many scientists in this country came 
to realize the need of organizing scientific research for 
service in a national emergency. Recommendations 
which they made to the White House were given care- 
ful and sympathetic attention, and as a result the 
National Defense Research Committee [NDRC] was 
formed by Executive Order of the President in the 
summer of 1940. The members of NDRC, appointed 
by the President, were instructed to supplement the 
work of the Army and the Navy in the development 
of the instrumentalities of war. A year later, upon 
the establishment of the Office of Scientific Research 
and Development [OSRD], NDRC became one of 
its units. 

The Summary Technical Report of NDRC is a 
conscientious effort on the part of NDRC to sum- 
marize and evaluate its work and to present it in a 
useful and permanent form. It comprises some 
seventy volumes broken into groups corresponding 
to the NDRC Divisions, Panels, and Committees. 

The Summary Technical Report of each Division, 
Panel, or Committee is an integral survey of the work 
of that group. The first volume of each group’s re- 
port contains a summary of the report, stating the 
problems presented and the philosophy of attacking 
them and summarizing the results of the research, de- 
velopment, and training activities undertaken. Some 
volumes may be “state of the art” treatises covering 
subjects to which various research groups have con- 
tributed information. Others may contain descrip- 
tions of devices developed in the laboratories. A 
master index of all these divisional, panel, and com- 
mittee reports which together constitute the Sum- 
mary Technical Report of NDRC is contained in a 
separate volume, which also includes the index of a 
microfilm record of pertinent technical laboratory 
reports and reference material. 

Some of the NDRC-sponsored researches which 
had been declassified by the end of 1945 were of 
sufficient popular interest that it was found desirable 
to report them in the form of monographs, such as 
the series on radar by Division 14 and the monograph 
on sampling inspection by the Applied Mathematics 
Panel. Since the material treated in them is not dupli- 


cated in the Summary Technical Report of NDRC, 
the monographs are an important part of the story 
of these aspects of NDRC research. 

In contrast to the information on radar, which is 
of widespread interest and much of which is released 
to the public, the research on subsurface warfare is 
largely classified and is of general interest to a more 
restricted group. As a consequence, the report of 
Division 6 is found almost entirely in its Summary 
Technical Report, which runs to over twenty volumes. 
The extent of the work of a Division cannot therefore 
be judged solely by the number of volumes devoted 
to it in the Summary Technical Report of NDRC: 
account must be taken of the monographs and avail- 
able reports published elsewhere. 

Any great cooperative endeavor must stand or fall 
with the will and integrity of the men engaged in it. 
This fact held true for NDRC from its inception, and 
for Division 6 under the leadership of Dr. John T. 
Tate. To Dr. Tate and the men who worked with 
him — some as members of Division 6, some as 
representatives of the Division’s contractors — be- 
longs the sincere gratitude of the Nation for a diffi- 
cult and often dangerous job well done. Their efforts 
contributed significantly to the outcome of our naval 
operations during the war and richly deserved the 
warm response they received from the Navy. In ad- 
dition, their contributions to the knowledge of the 
ocean and to the art of oceanographic research will 
assuredly speed peacetime investigations in this field 
and bring rich benefits to all mankind. 

The Summary Technical Report of Division 6, 
prepared under the direction of the Division Chief 
and authorized by him for publication, not only 
presents the methods and results of widely varied re- 
search and development programs but is essentially a 
record of the unstinted loyal cooperation of able men 
linked in a common effort to contribute to the defense 
of their Nation. To them all we extend our deep 
appreciation. 

Vannevar Bush, Director 
Office of Scientific Research and Development 

J. B. Conant, Chairman 
National Defense Research Committee 































































































































































































FOREWORD 


A very substantial portion of the research and 
development effort of Division 6 related to 
acoustic homing control of torpedoes and mines. This 
effort led to the design and production of homing 
devices which found important Service use. The 
British, likewise, undertook research and develop- 
ment in this same field, but the results of their work 
apparently found limited Service application. Also, 
the enemy was not idle, for somew T hat to our distress 
the Germans developed an acoustic homing torpedo; 
but, it should be added, reasonably effective counter- 
measures quite promptly became available — a fact, 
which, incidentally, is very pertinent to future con- 
sideration of devices of this type. 

When about a year ago the plans were laid for the 
Division 6 summary report series, there was proposed 
for this volume material relating to actual structures. 
However, because of pressure of their other duties, it 
has not been possible for the persons competent to 
summarize the mass of material involved to under- 
take this task. All of this material has, however, been 
made available to interested Service technical per- 
sonnel. Also, since the Division 6 report series was 


planned, the Office of Research and Inventions asked 
for a comprehensive study and report on torpedoes. 
As a part of this project there was furnished to that 
office a report on guided torpedoes, and presumably 
this also is available to interested Service personnel. 

This report will therefore be somewhat restricted 
in its scope. It includes, however, as Part I a general 
analysis of the problem presented, Principles and Ap- 
plications of Acoustic Homing Control to Torpedoes , 
prepared by Dr. W. V. Houston, and as Part II a re- 
port Echo-Ranging Torpedo Control Systems, by Dr. 
V. M. Albers. This second report is peculiarly perti- 
nent as it covers the less highly developed method of 
acoustic control and one which may hold considerable 
future promise. 

In the bibliography appears a list of the more 
pertinent reports prepared by Division 6. 

John T. Tate 
Chief, Division 6 

E. H. Colpitts 
Chief, Section 6.1 


VII 

























































































PREFACE 


Part I 


T his report was prepared by the Columbia Uni- 
versity Special Studies Group as part of the 
studies made in connection with Projects NO-94, 
NO-149, NO-157, and NO-181. It covers the sum of 
the theoretical studies associated with the applica- 
tion of acoustic control to torpedoes of various kinds. 

Although some of the theoretical work was done 
directly by the Special Studies Group, much of this 
report is a compilation of experimental results and 
theoretical developments carried on by the various 
other groups concerned. Some attempt has been made 
to give credit to these groups for their work, but this 
has not been possible in all cases. 


A major part of the report is concerned with 
torpedo self noise and its influence on control systems. 
Torpedo self noise appears to be the dominating factor 
that limits the use of such control systems so that 
methods for its control must constitute the principal 
objective of future research along these lines. 

This report contains reference to specific develop- 
ments only in so far as they are useful in illustrating 
the theoretical principles. Detailed descriptions of the 
various systems that were tried and of their success 
will be found elsewhere. 

W. V. Houston 


Part II 


This report is intended to cover, as far as possible, 
the work which has been done in the development of 
echo-ranging control for torpedoes. It is assumed 
that the reader is familiar with conventional elec- 
tronic circuits. The emphasis, in discussing the vari- 
ous systems, is on their functional behavior and on 
the details of circuit design which are unconventional. 

It is important for the reader to bear in mind the 
fact that all of the systems described, except the 
General Electric system, are actually still in the re- 
search stage and the General Electric system is, at 
the time of preparation of this report, just in the pre- 
production stage. The chapters covering the various 
British systems are necessarily very brief since rela- 
tively little material is available about them. 

The report is divided into four main parts. The 
first, which is essentially introductory, covers a re- 
view of terminology in underwater sound, the nature 
of the problem of echo-ranging torpedo control and a 
general description of the major components in- 
volved in all echo-ranging control systems. The 
second and third cover, respectively, the systems 


developed for antisubmarine and anti-surface-ship 
service with a separate chapter devoted to each 
system. The fourth division contains an attempt at 
evaluation of the work which has been done up to the 
time of preparation of this report. 

In discussing each of the systems a general descrip- 
tion is first given with a block diagram in order to 
indicate the general principles utilized in the device. 
This is followed, in all cases where the information is 
available, by a detailed discussion of the major 
components of the system with individual circuit 
diagrams of each functional component. 

This report is confined chiefly to the descriptions 
of the electronic gear which is used in the echo-rang- 
ing torpedoes since the application has so far been 
almost entirely to existing torpedoes which have been 
adequately described elsewhere. Only those physical 
characteristics which determine the behavior of a 
torpedo under echo-ranging control are noted. 

Vernon M. Albers 


IX 











































. 




















































CONTENTS 


PART I 

INTRODUCTION 
By W. V. Houston 

CHAPTER PAGE 

1 Objectives of Homing Control 3 

2 Properties of Acoustic Homing Systems 5 

3 General Observations on Torpedo Self Noise 7 

4 Cavitation and Cavitation Noise 10 

5 Machinery and Other Noise 18 

6 Hydrophone Discrimination and Isolation 23 

7 Total Torpedo Noise 25 

8 Transformation of Acoustic Signal into a DC Voltage 31 

9 Torpedo Dynamics and Stability 37 

10 Miscellaneous Problems 46 

11 Signal and Noise Levels 50 

12 Identification of the Echo 53 

13 Application of Echo to Torpedo Steering 56 

14 Needs for Further Study 58 

PART II 

ECHO-RANGING TORPEDO CONTROL SYSTEMS 
By Vernon M. Albers 

15 Introduction 63 

16 Major Components Used 70 

17 Nature of the Control Problem 72 

18 General Electric NO 181 System 76 

19 Harvard Underwater Sound Laboratory N0181 System 85 

20 The British Dealer System 104 

21 Ordnance Research Laboratory Project 4 System 105 

22 Bell Telephone Laboratories 157B and 157C S Systems ...... 120 

23 Geier Torpedo Control System 138 

24 British Trumper System 149 

25 British Bowler System 154 

26 Evaluation 156 

Glossary 161 

Bibliography 163 

Contract Numbers 167 

Service Project Numbers 168 

Index 169 


W M 












































' :n- ■ 






PART I 


INTRODUCTION 





































































Chapter 1 

OBJECTIVES OF HOMING CONTROL 


T he modern naval torpedo is a highly effective 
weapon by means of which a large explosive 
charge is detonated near the underwater part of the 
hull of an enemy ship. It is distinguished from a mine 
by the fact that it is self-propelled and can be di- 
rected toward a selected target. 

Traditionally the torpedo is a weapon of stealth. 
It can be launched several miles from the target and 
travels underwater along its predetermined course. 
In many cases the explosion gives the first indication 
of immediate danger, and it is often difficult to de- 
termine whether the ship has been struck by a tor- 
pedo or has itself struck a mine. 

However, the surprise feature of a torpedo attack 
is sacrificed when torpedoes are launched from air- 
planes. In such cases it is usually possible for the 
target ship to detect the launching of the torpedo and 
to take suitable evasive action. Even in the case of a 
submarine-launched torpedo, the wake produced by 
the ordinary steam-driven weapon and the noise 
made by the driving mechanism provide means by 
which the approach of a torpedo may be detected in 
time to take evasive action. This evasive action usu- 
ally takes the form of turning directly toward or 
away from the oncoming torpedo. Such a maneuver 
presents to the torpedo, or salvo of torpedoes, such a 
narrow target that the probability of a hit is very 
much reduced. Furthermore, since the speed at 
which a torpedo travels may not be much greater 
than that of the ship, almost any turn may upset the 
nice calculations on which the aiming of the torpedo 
was based. 

A simple torpedo is expected to follow a preset 
course at a specified depth, and the accuracy with 
which it does so is a measure of the effectiveness of 
the control mechanism. However, in order to deter- 
mine what course to set, it is necessary for the torpedo 
officer to know the speed, course, bearing, and range 
of the target ship. The speed and the course are often 
difficult to determine, especially on the basis of the 
restricted view available through the periscope of a 
submarine, so that it is not surprising that many 
torpedoes miss their targets, even when no evasive 
action is taken. This fact is so well recognized that it 
is customary to fire salvos of as many as four tor- 
pedoes on slightly different courses in order to cover 


the possible error in aim. This method has, of course, 
the additional advantage of being somewhat of a 
countermeasure to evasive action. 

The object of a homing device is to make possible 
a lethal hit by guiding the torpedo to the target in 
spite of the evasive action that may be taken and in 
spite of errors in the original set course of the torpedo. 

A homing device is intended to minimize the effect 
both of aiming errors and of evasive action taken by 
the target. Such a device makes use of some char- 
acteristic property of the target, so that when the 
torpedo comes within the homing range it no longer 
follows the preset course but is directed toward the 
target. In such a case the torpedo will eventually 
strike the target and explode, unless the speed of the 
target permits it to run away. No evasive action of 
the nature of changes in course is effective, since pre- 
sumably a torpedo can maneuver as well as any ship 
against which it would be directed. 

A homing device can be associated with various 
types of search procedure. One extreme is illustrated 
by the ExF42 mine and the ExFF3 torpedo. In these 
cases the torpedo is launched from the air or from a 
submarine and follows a circular path at the set depth 
until it picks up the target on which it then homes. 
The only information necessary for launching is that 
required to place the torpedo within its operating 
range of the target. This is quite feasible in the case 
of a torpedo launched from an airplane against a sub- 
marine that has been sighted on the surface, and 
possibly in some cases for a torpedo launched from a 
submarine against an attacking surface vessel. 

In most cases, however, the effective homing range 
will be much smaller than the running range, and a 
preset course must be used to direct the torpedo into 
the neighborhood of the target. In this latter case the 
torpedo is aimed in the usual way and the homing 
mechanism serves to correct the course, if necessary, 
near the end of the run. In this form, the effect of the 
homing device may be described as an enlargement 
of the target. If the homing range is 100 yd, the 
effective target may be roughly pictured as extend- 
ing 100 yd in all directions from the point of the 
target ship on which the device homes. This is, of 
course, only a crude way of looking at the matter. 
The effectiveness of each homing mechanism must be 


3 


4 


OBJECTIVES OF HOMING CONTROL 


worked out on the basis of its own particular prop- 
erties. 

The effectiveness of a homing torpedo depends 
principally on two features. 

1. The effective homing range should be as great 
as possible and should not be confined to the direc- 
tion straight ahead. Homing range straight ahead is 
of limited value, for the torpedo will strike what is 
straight ahead of it without any homing mechanism. 
On the other hand, too much homing sensitivity to 
the side may make the torpedo susceptible to certain 
types of decoy. Generally it seems desirable to have 
a long homing range rather uniformly distributed 
ahead of the torpedo. 

2. The turning radius of the torpedo must be suf- 
ficiently short. It is quite important that the turning 
radius be shorter than the homing range if it is to get 
around so it can approach the target at all. On the 
other hand too short a turning radius makes for in- 
stability along the course and leads to difficulties in 
the automatic steering. The torpedo must be ma- 
neuverable enough so that it can keep pointed at the 
target, but it must also be steady on the desired 
course. 

The advantages of homing devices are fairly ob- 
vious but it must not be forgotten that there are also 
disadvantages. Among them may be mentioned the 
following six. 

1. The homing mechanism occupies space and 
weight that must be taken from either the explosive 
charge or from the fuel supply. The advisability of 
doing this requires a careful consideration of all the 
facts, with particular emphasis on the properties of 
the targets against which the weapon is likely to 
be used. 

2. The homing mechanism introduces increased 
complication in manufacture and maintenance. The 
techniques involved in the homing devices may be 
quite different from those normally associated with 
torpedoes, so that a completely new type of training 


may be necessary for adequate maintenance and 
operation. In large-scale planning this may be a very 
significant factor. 

3. To gain an adequate homing range it may be 
necessary to operate at a lower torpedo speed than 
would otherwise be possible. In the case of acoustic 
homing it is the self noise of the torpedo that usually 
sets the limit to the homing range, and this noise in- 
creases rapidly as the torpedo speed increases. It 
must then be decided whether the necessary decrease 
in speed is justified by the advantages of the homing 
property. This decision involves a knowledge of the 
speed of the expected target ships, since obviously it 
would be useless to use a torpedo too slow to reach 
the target attacked. 

4. Different homing systems sometimes tend to 
cause the torpedo to strike different parts of the ship. 
An acoustic torpedo that operates by listening tends 
to end its course in a stern chase and to strike near 
the propellers. An echo-ranging torpedo may, under 
some circumstances, tend to strike near the bow. 
These points of impact may not always be satisfac- 
tory since, although a hit on the propellers may be 
disabling, it may not be correct to assume that the 
ship will usually sink. 

5. Most homing devices are subject to some form 
of more or less effective countermeasure which, if 
used, may make the torpedo less likely to hit the 
target than if the homing device had not been 
present. 

6. Homing devices may limit the number of tor- 
pedoes which can be fired simultaneously without 
mutual interference. This is particularly true of 
acoustic listening torpedoes that may have a con- 
siderable homing range on each other. 

In spite of these limitations it appears that the 
homing devices now known are definitely an advan- 
tage to the group using them since, in general, the 
number of torpedoes that must be launched to make 
a hit is markedly reduced. 


Chapter 2 

PROPERTIES OF ACOUSTIC HOMING SYSTEMS 


"arious types of homing control have been sug- 
gested. Radio control would be convenient be- 
cause of the extensive development of radio tech- 
niques. However, no radio waves of usable frequency 
are known that will penetrate water more than a very 
short distance. For this reason such a torpedo would 
have to be equipped with an antenna projecting out 
of the water. Such an appendage would seriously 
hamper the motion and steering of the torpedo and 
does not seem to be very practicable. On the other 
hand, torpedoes have been built with such antennas 
so they can be radio-controlled from a launching 
plane. However, they have not been extensively used. 

Magnetic control has been suggested and may be 
practicable, but the intensity of the magnetic dis- 
turbance due to a target ship falls off so rapidly with 
distance that it seems very doubtful if homing ranges 
as great as 100 yd could be obtained. However, if 
countermeasures against acoustic homing devices 
appear to be highly effective, the development of 
magnetic homing may be justified. 

It has also been suggested that some method of 
following up the wake of a ship could be used to pro- 
duce a homing control. It is possible that such a de- 
vice would operate satisfactorily, but its tactical use- 
fulness would be somewhat limited. It would have to 
be fired so as to come in contact with the wake at 
some point not too far behind the ship, and it could 
not be used against a stationary ship. Furthermore 
in the case of a maneuvering target, the wake might 
be such a complicated affair that it would be difficult 
to follow. Nevertheless such a homing device may 
have its uses, although it has apparently not been de- 
veloped sufficiently so that the results of extensive 
field tests are available. 

The most promising type of homing control, and 
the one that has been the object of extensive study 
and development during the past four years is acous- 
tic. Sound travels in water without too much attenu- 
ation as long as the frequency is below some 60,000 c. 
Although the ocean is not a homogeneous medium, 
and the sound traveling in it may be reflected, re- 
fracted, and scattered, sound signals can be sent for 
appreciable distances. For about thirty years sonic 
methods have been used for locating submarines, for 
communicating between submarines, and for com- 
municating between submarines and surface ships. 


In this connection much has been learned about the 
propagation of sound in sea water, and during the 
last few years this study, and the study of acoustic 
homing devices, has been carried to such an extent 
that it is possible to lay down in a general way the 
possibilities and the limitations of acoustic homing 
devices for subsurface use. This is not to say that no 
more research is needed, but the lines along which 
research needs to be done can be pretty well laid 
down. 

Methods for sonic location of a target can be di- 
vided into two classes and homing devices based on 
each have been built. 

1. Echo ranging. This is a method of determining 
the direction and distance of a reflecting body by the 
echo it sends back in response to a sound signal. The 
homing device must then respond to the echo in 
such a way as to direct the torpedo toward it. 

2. Listening. In this method the sound travels in 
one direction only. To operate a listening method of 
homing control the target must produce noise. This 
noise is then picked up by the homing device which 
determines the direction from which the signal is 
coming and directs itself toward the target. 

Both of the above methods have been tried for 
homing torpedoes and have been shown to work sat- 
isfactorily within certain limits. The listening method 
is the simpler, not only in its acoustic and electronic 
gear, but in the application to the torpedo controls 
of the information received. On the other hand it re- 
quires that the target ship make some noise in the 
adopted frequency range, and hence it is ineffective 
against a ship at rest and quiet. In connection with 
torpedoes for use against submarines, it appears that 
a submarine at considerable depth makes practically 
no noise, since the cavitation of its propellers is sup- 
pressed and it may be very quiet in the frequency 
ranges normally used. In addition, a listening device 
may be countered by the operation of a strong source 
of noise at a distance from the target ship. Such a 
decoy can be built to simulate a ship’s noise very 
closely. 

The echo ranging type of homing control is effec- 
tive against a stationary target as well as a moving 
target, and would be effective against a deeply sub- 
merged submarine. To counter it appears to require 
some method of producing false echoes such as an 


5 


6 


PROPERTIES OF ACOUSTIC HOMING SYSTEMS 


echo repeater. It is, however, more complicated in 
construction and the exact extent to which it is effec- 
tive has not yet been determined in operational use. 

The operation of either type of acoustic homing 
torpedo is limited by the conditions of the water. For 
the ranges normally under consideration, 100 to 
1,000 yd, this is only serious under the worst condi- 
tions. Nevertheless it appears that such regions as 
Chesapeake Bay in the summer are so bad that echo- 
ranging operation over as much as 500 yd is im- 
probable. Although both types of homing control are 
limited by refraction of sound in the water, the echo- 
ranging type is, in addition, seriously handicapped 
by reverberation and false echoes. Echoes from the 
bottom are so serious that satisfactory operation in 
shallow water is unlikely, and even reflections from 
the surface may occasionally be troublesome. 

The operating range of any acoustic torpedo is 
most seriously limited by the torpedo self noise. This 
noise may be generated in the water because of cavi- 
tation of the propellers, cavitation on various parts 
of the body, and other causes, or it may be the noise 
made by the torpedo machinery. The noise due to 
cavitation can be reduced or eliminated by running 
the torpedo at a sufficiently great depth of submer- 
gence. If the torpedo is to be used as an antisub- 
marine weapon this is no additional complication, 
but if it is to be used against surface ships, the neces- 
sity of arranging for the torpedo to rise in the proper 
way to strike the target introduces numerous prob- 
lems. 

In any case the torpedo self noise increases with its 
speed, and it is probably safe to assert that the 
acoustic operating range of a high-speed torpedo can 
never be made so great as that of a slower torpedo. 
Nevertheless it may still be great enough to be use- 
ful. A high-speed torpedo is needed only in case the 
target is a high-speed vessel. The high speed of the 
target causes it to produce more noise than a lower- 
speed vessel and so to some extent counteracts the 
increased self noise of the torpedo. This advantage 
does not accrue in the case of an echo-ranging control 
where the range is not increased by an increase in 
target noise. 

The study of torpedo self noise and of means for 


reducing it constitutes the principal line of effort in 
the improvement of acoustic homing torpedoes. Two 
lines of attack are possible. One is the reduction of 
the noise at its source. This involves a study of pro- 
peller design and the general overall shape with the 
object of eliminating cavitation and reducing other 
types of water noise. It also involves the reduction 
of internal machinery noise due to gears, flow of high 
pressure air, and motion of high-speed parts. 

The other line of attack is the attempt to reduce 
the hydrophone response to the noise that exists. 
This involves the selection of hydrophones whose 
directivity pattern is such that there is very little 
response to cavitation and other noise produced in 
the water. It also involves mounting the hydrophone 
in such a way that vibrations are not picked up from 
the body of the torpedo itself. In addition it may be 
helpful to modify the torpedo structure at various 
points in order to reduce the transmission of sound to 
the shell and through it to the hydrophones. 

Much work has already been done along these 
lines but much more remains to be done before the 
best practical acoustic homing torpedo can be built. 

A consideration of the importance of torpedo speed 
in reaching the target, and of self noise in reducing 
the homing range suggests that the ideal torpedo 
would have at least two, and possibly three, speeds. 
The maximum speed would be used for attacking the 
fastest ships. The self noise of the torpedo would be 
fairly high but the high noise of the target ship would 
override this in a listening torpedo, and in an echo- 
ranging torpedo some reduction in range would have 
to be accepted since the torpedo must have enough 
speed to catch the target. 

Against lower-speed targets a lower torpedo speed 
could be used. The speed-changing mechanism would 
also increase the acoustic sensitivity to make use of 
the reduced self noise and so keep the effective acous- 
tic range from being seriously reduced by the re- 
duction in target noise. 

It seems possible that careful selection of the vari- 
ous speeds and the best utilization of them might 
lead to the specification of an almost universal tor- 
pedo for use from submarines and possibly another 
for use from airplanes. 


Chapter 3 


GENERAL OBSERVATIONS ON TORPEDO SELF NOISE 


T o operate properly, an acoustic torpedo must 
distinguish between the signal on which it is ex- 
pected to steer and other noises that may be present. 
These other noises may be in the surrounding water 
or they may be in the torpedo itself. Careful measure- 
ments, made by the Bell Telephone Laboratories 
[BTL] 5 in rather deep water, indicate that far away 
from the shore the water background noise is princi- 
pally due to surface disturbances such as whitecaps. 
It was found that when no whitecaps were present, 
the noise level at 25 kc was as low as —74 dbs, a 
while with many whitecaps it was as high as — 50 dbs. 
Heavy swells seemed to have no effect in producing 
noise. 

Water noises provide a limit below which no acous- 
tic torpedo can be expected to operate. However, if 
the water noise is isotropic, i.e., if it comes equally 
from all directions, a directional hydrophone will re- 
spond to it very much less than to a plane wave com- 
ing along the direction of maximum sensitivity. The 
difference in response to these two types of sound is 
just the directivity index of the hydrophone. Because 
directional hydrophones are normally used, as well 
as because a torpedo is normally a rather noisy ma- 
chine, the water noise is usually well below the tor- 
pedo noise and can be neglected. 

3.1 IMPORTANCE OF SELF NOISE 

This noise of the torpedo itself, as it affects the 
hydrophones in the torpedo, is normally the most 
important limiting factor in the operation of an 
acoustic torpedo. This is quite clear in the case of a 
torpedo that listens to the noise of the target ship. 
A torpedo has most of the characteristics of a ship, 
especially a submarine, on a small scale; and it is not 
likely that any significant difference in quality will 
exist between the noise of the target and the noise of 
the torpedo. Hence the discrimination must be made 
on the basis of intensity alone. When this is done, it 
is a fair general statement to say that acoustic con- 
trol can be expected only when the root mean square 

a The abbreviation “dbs” signifies decibels spectrum level 
and refers to the intensity of the noise in a frequency band 
1 c wide. The reference intensity is that corresponding to a 
root mean square pressure of 1 dyne per sq cm. For a discus- 
sion of terminology and reference levels in sound measure- 
ments see Division 6, Volume 10, Calibration Methods, Chap- 
ter 4, “Types of Acoustic Measurements.” 


[rms] value of the target noise is equal to or greater 
than the rms response of the hydrophone to the tor- 
pedo self noise. b Sometimes it is possible, by special 
arrangements, to recognize a slightly lower signal, 
but frequently it is considered that operation is re- 
liable only when the signal is somewhat above the 
self noise. Nevertheless, the above statement is a 
satisfactory rough criterion for estimating the acous- 
tic operating range of any listening control. 

A possible exception to the general statement may 
be the case in which the target noise is strongly mod- 
ulated with the period of the propeller blade fre- 
quency. Since the top of a large ship propeller is 
nearer the surface of the water than the bottom, 
each blade may have its maximum cavitation as it 
passes through the top position. Since such propellers 
usually have a much smaller rate of rotation, the mod- 
ulation frequency is lower and the target noise may be 
identified at a slightly lower level than indicated. 

With an echo-ranging control the problem appears 
at first to be a little different. It may be possible to 
impose on the emitted signal such a character that it 
can be detected in the presence of noise. However, 
the advantage to be gained by this method is dis- 
tinctly limited because the target is rarely a plane 
reflecting surface. In general it is made up of a num- 
ber of surfaces and the result of their joint action 
tends to destroy the character of the incident ping 
and make it more like noise. 

Hence for both types of acoustic control, it may be 
stated as a general approximate rule that for success- 
ful operation the rms response of the hydrophones to the 
torpedo self noise must he less than , or at most equal to, 
the rms response to the signal from the target. 

Since in a listening torpedo the signal strength is 
fixed by the nature of the target and in an echo- 
ranging type it is not practical to increase the signal 
strength indefinitely, attention must be given to re- 
ducing the response of the hydrophones to the tor- 
pedo self noise. The condition is worded this way be- 
cause it can be accomplished by (1) reducing the 
level of the noise generated and (2) reducing the 
sensitivity of the hydrophones to the predominant 
sources of noise. In order to apply either of these 

b There is a possibility that characteristic modulations of 
noise may be present which could be used to give added 
discrimination. 


7 


8 


GENERAL OBSERVATIONS ON TORPEDO SELF NOISE 


methods, it is important both to identify the sources 
of noise and to study the means by which the noise 
is transmitted to the hydrophones. 

3.2 EXTERNAL MEASUREMENTS OF 
TORPEDO NOISE 

The simplest way of getting a preliminary estimate 
of torpedo noise is to measure it with a hydrophone 
fixed in the water, as the torpedo runs past. Possibly 
the principal difficulty in such a measurement lies in 
determining with sufficient accuracy the distance 
between the hydrophone and the torpedo that corre- 
sponds to the measured noise levels. Nevertheless 
measurements made in this way serve as a starting 
point for the general study of the problem, and in 
fact observations of different observers show a re- 
markable amount of agreement. 

Figure 1 shows a series of curves based on the 
measurements of a number of different observers. 1 



0 10 20 30 40 50 60 

FREQUENCY IN KC 

Figure 1 . Noise level measurements on various torpe- 
does at a distance of 6 meters. These curves represent 
averages of a number of observers. 

Although the observers differ somewhat among them- 
selves, and in all probability the conditions of meas- 
urements were not identical, there is sufficient agree- 
ment among them to permit drawing the rough 


average curves shown in the figure. From these 
curves a number of conclusions can be drawn. 

1. Individual differences between different kinds 
of torpedoes seem relatively insignificant. In par- 
ticular the Mark 18 electric torpedo is no quieter 
than the Mark 13 turbine-driven torpedo. 

2. It is possible to plot a curve of torpedo noise as 
a function of speed at a given depth. This is done in 
Figure 2 which shows the noise level, at 25 kc, as a 
function of speed for torpedoes running 12 to 15 ft 



SPEED IN KNOTS 

Figure 2. A curve of noise level at 25 kc as a function 
of speed, for an “ideal” torpedo. The points indicated 
were taken from the curves in Figure 1. 

deep. A smooth curve drawn through these points 
may then be regarded as the noise-speed curve of an 
idealized torpedo, and it seems possible to discuss 
the properties of this ideal torpedo. This lumping of 
all torpedoes together must be viewed with some 
caution. The observations used refer to British, 
American, and one German torpedo only, and it may 
be incorrect to extend the generalization to other 
foreign torpedoes. It is possible that the British and 
American trends of development have been such as 
to lead to torpedoes that are very similar in their 
noise characteristics above 10 kc. Figure 3 shows the 
curve of Figure 2 with a large number of individual 
observations indicated. This shows that the disper- 
sion of the measurements on any one type of torpedo 
is at least as great as any possible differences between 
different torpedoes. It is clear that the curve drawn 


EXTERNAL MEASUREMENTS OF TORPEDO NOISE 


9 


is not the one that would be drawn through the 
points if they were all given equal weight. However, 
if the observations for the same type of torpedo are 
first put together as for Figure 2 the fit is fairly good. 


1 : 1 1 

A BRITISH MARKS 3ZH1 AND IX 
© BRITISH MARK XV 
+ US MARK 13 
□ US MARK 14 




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*RK 18 

H MARK X 
WELECTR 

i 

IC(G7E) 

0 \ 

□ 

A 




A 






< 

©< 

m 

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V 

/ cr - 





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SPEED IN KNOTS 

Figure 3. This curve is the same as in Figure 2 but 
the points represent a number of individual observa- 
tions. The distribution of these points gives an idea 
of the extent of agreement between different observers. 
The curve is not intended to be the best curve through 
the points but is based on illustrative assumptions used 
in Chapter 9. 


3. This ideal curve of noise at 25 kc seems to rise 
rather rapidly between 20 and 30 knots and there- 
after to rise more slowly. As will be indicated in the 
next chapters this rapid rise probably represents the 
development of propeller and other cavitation, which 
is the dominant source of noise in this region. Above 
30 knots the machinery noise becomes dominant 
again and continues to rise, while the cavitation 
noise remains constant or even falls off a little. Below 
20 knots it is probable that individual differences 
between torpedoes make it more difficult to make the 
same kinds of generalization. It is also probable that 
at lower frequencies the individual characteristics of 
torpedoes are more significant. 

4. The noise level of all torpedoes seems to fall off 
roughly 6 db per octave. This indicates that the 
noise is inversely proportional to the square of the 
frequency, above 10 kc. This again is in rough agree- 
ment vith observations on ships, so that there is 
probably no advantage from this point of view in 
selecting one frequency rather than another. Al- 
though the self noise falls off as the frequency is in- 
creased, the target noise, for a listening torpedo, falls 
off also, and at about the same rate. Two other fac- 
tors, however, need some consideration. One is that 
the hydrophone discrimination to be discussed in 
Chapter 6 probably increases as the frequency rises 
so that the use of a higher frequency may produce an 
improvement in signal-to-noise ratio. On the other 
hand, the attenuation increases rapidly with fre- 
quency from somewhere around 4 db per kilometer 
near 25 kc to around 14 db per kilometer near 60 kc. 

The question of the best frequency to use has not 
been at all thoroughly investigated. In the following 
discussion of self noise attention will be centered on 
the 25-kc region, since most of the work has been 
done there. 


Chapter 4 

% 

CAVITATION AND CAVITATION NOISE 


4.1 THE NATURE OF CAVITATION 

T he phenomenon of cavitation gets its name from 
the fact that there is a production of actual cav- 
ities in the water. There may be a few large cavities 
or a large number of very small cavities, either at- 
tached to the surface of moving bodies or free in the 
water stream itself. Noise is nearly always associated 
with the phenomenon of cavitation. The exact mech- 
anism is still obscure, but either the production of the 
cavities, the vibration of the cavities, or possibly the 
collapse of these cavities as they move into a region 
of higher pressure provides one of the most important 
sources of the underwater noise associated with a 
torpedo. For this reason, it is important to have some 
knowledge of the conditions under which cavitation 
occurs. 

Cavitation is essentially a phenomenon of liquids. 
It occurs at points in the liquid where the pressure of 
a perfect incompressible fluid would become nega- 
tive. In reality, it is only necessary that the pressure 
fall below the vapor pressure of the liquid, for then 
the liquid opens up in small cavities that are immedi- 
ately filled with vapor. On this picture, it is clear 
why cavitation does not occur in gases. If an attempt 
is made to reduce the pressure of a gas below zero, 
the gas merely expands. No matter how much the 
volume is enlarged, the gas continues to expand and 
fills the whole available space. Water, on the con- 
trary, does not expand very much when the pressure 
is reduced, but instead it evaporates to fill the avail- 
able volume with water vapor. In many cases the 
pressure of water vapor is insignificant compared 
with other pressures involved. In such cases the 
vapor pressure may be neglected, and it may be con- 
sidered that an attempt to reduce the pressure of a 
liquid below zero results merely in the creation of a 
vacuums 

4 . 1.1 A Simple Case of Cavitation 

A simple case of cavitation can be observed when 
water flows through a tube of diminishing cross sec- 

a It is true that under suitable conditions, liquids such as 
water can support a considerable tension. Under the circum- 
stances with which we are concerned, however, in which the 
water is not particularly pure and in which solid surfaces are 
moving through the water, this possibility of tension may be 
ignored, and it may be considered that a reduction of pressure 
to the vapor pressure of the water or below will always result 
in cavities. 


tion. Since the cross section is diminishing, the 
velocity of the water is increasing and, according to 
Bernoulli’s theorem, 

V + ipv 2 = constant. (1) 

The pressure must decrease as the cross section di- 
minishes because the pressure must decrease as the 
velocity increases. In equation (1) the pressure is 
measured in pounds per square foot, the density in 
slugs per cubic foot, and the velocity in feet per 
second. In these units, the density of water is ap- 
proximately 2, so that a velocity of 50 fps corre- 
sponds to a reduction of pressure of about 2,500 psf, 
or more than an atmosphere. If the pressure is one 
atmosphere where the water has a negligible velocity, 
cavitation will occur down the stream where the 
velocity approaches 50 fps. 

4.1.2 Vortex Cavitation 

Another situation under which cavitation may 
occur is associated with rotational motion of the 
water and the accompanying centrifugal force. As 
an illustration of the kind of thing that can occur, 
one may consider a simple vortex, at the center of 
which pressure will be a minimum. This can be un- 
derstood by considering an idealized vortex consisting 
of the rotation of a cylindrical volume of water whose 
radius is R, at a constant angular velocity co. The 
velocity at any point inside the cylinder is then 
given by 

v = a >r, 

where r is the distance from the center of the cylin- 
der. Outside of this cylinder the motion is “irrota- 
tional,” even though the water is moving about the 
central cylindrical core. This is possible if the velocity 
is inversely proportional to r. These two motions give 
the same velocities at the radius R, if the constants 
are properly selected. Hence let us assume that 



I 


for r < R, 

v = cor = l 

,2tt R 2 ) r ’ 


0)R 2 

K 

for r > R, 


2 irr 

r 


The significant measure of the strength of this vortex 
is the quantity K that is called the circulation. 


10 


THE NATURE OF CAVITATION 


II 


Outside the central cylinder of radius R the motion 
is “irrotational,” and Bernoulli’s theorem can be 
used to determine the pressure in terms of the pres- 
sure at great distances from the vortex. This leads to 

e(k Y 

P = — ^V 2 = Poo 2\27! 7 

2 

For r < R, the analysis shows that 

P + ^ = P»-p (^) 2 + p*A 


whence 


V = Poo 


/ K V p/_K_V 

\2xfl/ + 2\2irR-) 


dr 2 


At the center of the core the pressure is 
Po = P ”- p (2^) 2 - 


(2) 


This pressure depends both on K and R. It depends 
both on the strength of the vortex K and the extent 
of the vortex R. For a given value of K, the pressure 
becomes lower if R is made smaller. It is clear from 
this form of expression that it is quite possible for the 
pressure po at the center of the vortex to become 
negative, and hence for these equations to fail to 
describe the physical situation. Under such condi- 
tions, a cavity will open up at the center, and cavita- 
tion will occur. Figure 1 shows the way in which the 



Figure 1. Speed of the water and the pressure plotted 
as a function of the distance from the center for an ideal- 
ized vortex of a simple type. 

pressure and the velocity depend on the distance 
from the center of a simple vortex of this kind. 

The importance of cavitation in a vortex, when 
considering the self noise of a torpedo, comes from 
the fact that a trailing vortex is left behind the tip of 


a propeller that is driving a torpedo through the 
water. If the circulation around this vortex is great 
enough, or if the vortex is concentrated enough 
(R small enough), cavitation will occur and will be a 
serious source of noise. Since the magnitude of the 
propeller thrust is associated with the circulation K, 
the only way to avoid cavitation and to maintain the 
thrust is to shape the propeller blade so that the 
vortex is not shed in a concentrated form but is 
spread out over a considerable volume. This means 
that the vortex will have very little similarity to the 
simple type of vortex described here, but will be much 
more complex. Nevertheless, the principle involved 
is that illustrated; since the vorticity cannot be re- 
duced without reducing the thrust, efforts to reduce 
the cavitation of the propeller by suitable design 
must be directed toward getting the vorticity to 
occupy a volume greater than some minimum vol- 
ume. A somewhat intensive study of this problem 
has been made at the Harvard Underwater Sound 
Laboratory [HUSL]. 22 

Figure 2 shows a test propeller in a water tunnel at 
the David Taylor Model Basin [DTMB]. The cavi- 
tating vortices from the propeller tips can be clearly 
seen extending in helices down the stream. Only be- 
cause the vortex is ca vita ting can it be seen. 

4.1.3 Body Cavitation 

When a solid body is moving through the water 
with a velocity V, the speed of the water with refer- 
ence to the body has a variety of values at various 
points and at some places is considerably higher than 
V. At such points the pressure is lower than in the 
free water and may become low enough to initiate 
the phenomenon of cavitation. In this way, cavita- 
tion may occur in the neighborhood of a torpedo 
moving through the water quite apart from the cavi- 
tation that may exist in the propeller tip vortices. 

As an illustration, consider the case of a sphere. 2 
This is simple enough so that the flow of perfect 
fluid around it can be calculated. If the sphere is con- 
sidered to be at rest and the fluid to be moving past 
this with a velocity V, the velocity at a point in the 
fluid outside the sphere is 



The polar coordinates r and 0 are based on the center 


12 


CAVITATION ANI) CAVITATION NOISE 


of the sphere as the origin and the direction of flow 
as the polar axis. R is the radius of the sphere. 

It follows from these equations that the point of 
maximum velocity is at the surface of the sphere, 
r = R, where 6 = ir/2. At this point v 2 = 9F 2 /4. 
Then according to Bernoulli’s theorem, 

V + ^pV 2 = Poo + i pT 2 > 

and 

V - P- - gPt 72 - (4) 

This gives the pressure around the equator of the 
sphere in terms of velocity V, as long as the motion 
of the water is described by the above expression. 
When the velocity is large enough, so that p becomes 
equal to the vapor pressure of the water, or approxi- 
mately zero, cavitation will occur around the equator 
of the sphere. If the velocity is increased still further, 
the cavitation spreads over more and more of the 
sphere, and the equation ceases to give the correct 
description of the velocity of the water in the cavi- 
tation region. The type of calculation just indicated 
often serves to indicate the velocity at which cavi- 
tation will set in, but it does not serve to describe the 
further development of the phenomenon and the en- 
largement of the cavity. 

4 . 1.4 Cavitation Coefficient 

If V c is the velocity at which the cavitation just 
begins, the cavitation properties of the body may be 
described by a cavitation coefficient K c . From equa- 
tion (4) describing the pressure around the sphere, 



This value of K c = 1.25 may be regarded as a state- 
ment of the susceptibility of the equator of the sphere 
to cavitation. It is important to notice that V c is the 
velocity of the whole water stream with reference to 
the sphere and not the velocity of the water at the 
point where the cavitation begins. 

Calculations of this type have been worked out to 
give the critical values of K c for a number of different 
shapes. For ellipsoids of revolution whose major and 
minor axes are a and b, the following values were 
found to hold. 

a K e 

b 

1 1.25 

2 0.46 

3 0.26 

4 0.19 


In addition the value for a simple, “half streamline” 
body turns out to be 0.33. These results indicate that 
in general, cavitation occurs more readily around a 
blunt body than around a somewhat pointed body. 

4 . 1.5 Propeller Blade Cavitation 

A similar type of cavitation may also occur on the 
blades of a propeller, since these blades move through 
the water with a velocity considerably higher than 
the forward motion of a torpedo itself. A propeller 
blade is essentially a short hydrofoil moving through 
the water with a certain angle of attack, so that the 
pressure is increased on its face and decreased on its 
back. As the speed increases, this decrease of pres- 
sure becomes greater and greater and may eventually 
produce a pressure as low as the vapor pressure or 
lower. Under these circumstances, cavitation sets in 
as usual. This type of cavitation has been known for 
some time in connection with propellers, because it 
seriously reduces the efficiency of the propeller. The 
pressure of the water at some distance from the pro- 
peller blade provides a limit to the pressure reduc- 
tion on the back of a propeller blade. If this limit is 
reached at one point on the surface of the propeller, 
further increase in propeller speed produces less addi- 
tional thrust than it would otherwise do. 

It is not difficult to calculate the pressure distri- 
bution over simple hydrofoils, but it is somewhat 
more difficult to make the calculation for an actual 
propeller. Nevertheless, after a propeller is built, it 
is possible to test it for cavitation and to observe the 
conditions under which the phenomenon occurs. 
Cavitation is of importance in ordinary propellers 
because of its effect on propeller efficiency, but it is 
far more important in the case of acoustic torpedoes 
because of the noise that it produces. This has led to 
an increased interest in the study of this phenomenon 
and of methods of reducing it. As will be shown later, 
however, to eliminate the noise it seems necessary to 
essentially eliminate the cavitation, not merely to 
reduce its amount. 

4.2 OBSERVATIONS OF CAVITATION 

Figure 2 shows the way in which propeller tip 
cavitation can be observed in a water tunnel. If 
cavitation does not appear under normal operating 
conditions, it can usually be brought about by in- 
creasing the propeller loading or reducing the water 
pressure. 

Extensive observations of body cavitation have been 
made in the High Speed Water Tunnel at the Cali- 


OBSERVATIONS OF CAVITATION 


13 


fornia Institute of Technology [CIT]. 3 A small body 
of the shape to be studied was placed in the water 
stream and observed visually. Both the velocity of 
the water and its pressure could be varied, and it was 



Figure 2. Photographs of a propeller in a water tunnel 
at the David Taylor Model Basin showing the cavitat- 
ing vortices trailing from the propeller tips. 

shown that a cavitation coefficient K c , as defined 
above, could be attributed to various points on the 
body and would describe the conditions under which 
cavitation would appear. 

In a slightly different sense a cavitation parameter 
can be used to describe the conditions under which a 
body is moving, or the conditions in the water tunnel. 
In this case the velocity is just the existing velocity. 
It can then be said that cavitation begins at a certain 
point on the body when the cavitation parameter is 
decreased below the value of the cavitation coeffi- 
cient at the point. 

Observations on a sphere gave K = 1.2 as the 
point at which cavitation first set in. This is in ade- 


quate agreement with the calculated value 1.25. On 
a cylindrical body with a hemispherical nose, cavi- 
tation was observed for A about 0.75. This observed 
value of 0.75 indicates that a torpedo of this shape 
running 15 ft deep, where the pressure is approxi- 
mately 3,000 psf, would begin to cavitate at speed 
around 68.9 fps, or approximately 38 knots. 

Since it may well be desired to run acoustic tor- 
pedoes at speeds greater than 38 knots, and possibly 
at depths less than 15 ft, it is important to have nose 
shapes that do not cavitate so easily. A number of 
other shapes were tried in the Water Tunnel, with 


the following results. 

Nose length 
Body diameter 

K c 

Hemisphere 

0.50 

0.89 

Ogive 1.125-cal radius 

0.94 

0.41 

Ogive 2.0-cal radius 

1.33 

0.33 

Ellipsoid 

0.75 

0.48 

Ellipsoid 

Half streamline with quartic transition 

1.25 

0.30 

curve 

1.25 

0.35 


It must be noted that these values of the cavita- 
tion coefficient apply only when the torpedo is travel- 
ing in the direction of its axis, i.e., when the pitch 
and yaw are zero. At other values of the pitch and 
yaw cavitation tends to set in earlier. The elongated 
noses, in particular, are especially sensitive to yaw 
angles. Furthermore, other points on the torpedo 
show more pronounced tendencies to cavitate than 
the nose. In particular, the fins of the Mark 13 tor- 
pedo show a critical value of K c = 0.93 at zero yaw, 
and K c = 1.6 at 4 degrees yaw. Hence the problem 
of designing a torpedo to travel at high speeds with- 
out cavitation presents a number of difficulties. The 
torpedo will nearly always yaw and pitch to some 
extent, and it will certainly travel with a yaw angle 
if it is expected to turn in a small circle. Those shapes 
that tend to postpone cavitation to high speeds when 
traveling in a straight line tend also to produce cavi- 
tation easily when moving at an angle to their axes. 

Observations were also made at the High Speed 
Water Tunnel concerning the cavitation on an airfoil 
section. 3 They show quite clearly the dependence of 
the critical cavitation parameter on the angle of 
attack, and hence on the thrust exerted by a pro- 
peller. 

Studies have also been made by HUSL of propeller 
cavitation in a model propeller tunnel. This work 
showed that the incidence of cavitation could be 
described in terms of a cavitation parameter in the 
same way as cavitation about a torpedo nose. There 


14 


CAVITATION AND CAVITATION NOISE 


are various cavitation parameters that might be used 
in connection with a propeller. One of these uses the 
propeller tip speed as the significant velocity. On the 
other hand, it is true, of course, that those parts of 
the propeller blade close to the axis do not have so 
high a speed as the tips and hence the speed of the 
hydrofoil section is not always the same as tip speed. 
Another method is to use the forward speed of the 
propeller through the water. This at least is the same 
for all parts of the propeller and is roughly propor- 
tional to the propeller tip speed. It is probably of 
little importance which cavitation parameter is used, 
as long as its significance is recognized. If the forward 
speed through the water is used, cavitation will occur 
at rather large values of the cavitation parameter, 
because the propeller tips will be moving at a con- 
siderably higher speed than the speed used in the 
parameter. 

The HUSL observations indicated that propeller 
tip cavitation usually set in first, and that later cavi- 
tation on the blades could be observed. Similar ob- 
servations have been made at the DTMB propeller 
tunnel. 

4.3 NOISE DUE TO CAVITATION 

The principal importance of cavitation in the de- 
sign of acoustic torpedoes is due to the noise pro- 
duced. It is now quite clear that cavitation, even 
barely incipient cavitation, is a major source of noise, 
but the detailed mechanism of its production is not 
at all clear and much work remains to be done before 
quantitative relationships can be established between 
the intensity and character of the noise and the 
properties of the cavitation. 

When the work on underwater sound was first in- 
augurated in the University of California laboratory 
at Point Loma, some preliminary exploratory work 
was started at Berkeley. 4 It was hoped in this way to 
get some general indication as to the way noises are 
produced under water. The general conclusion was 
that the noise associated with breaking the surface of 
the water or with cavitation was greater in order 
of magnitude than that produced in any other way. 
The noise produced by moving rough surfaces 
through the water was almost negligible compared 
with that due to any kind of a surface disturbance. 
This is in conformity with general observation that 
ambient noise in the sea increases sharply as white- 
caps appear. 5 

In an effort to make a start on a study of the rela- 


tionship between cavitation and noise, a hydrophone 
was installed in the High Speed Water Tunnel at 
CIT. With this hydrophone, observations were made 
of the noise level as a function of the cavitation pa- 
rameter for a number of different torpedo and bomb 
models. This work is described in detail in a report 
from this laboratory. 6 

One of the major difficulties in the use of a water 
tunnel for noise measurements of this kind is the 
large amount of noise produced by the tunnel itself. 
The pumping machinery is noisy, and the noise is 
easily transmitted to the working section. This back- 
ground noise also varies with the speed and pressure 
of the water. Nevertheless, it was found in confirma- 
tion of the expectations, that the noise produced by 
cavitation on a model so outweighs the other noises 
that it can be clearly recognized. 

Another difficulty is the fact that the cross section 
of the tunnel in the working region is rather small. 
The measurement of sound intensity in such a space 
is difficult because of interference phenomena and 
reflection from the walls. The intensity measured is 
strongly dependent upon the exact position of the 
hydrophone as well as on its orientation. It is practi- 
cally impossible to get a significant measure of ab- 
solute sound intensity under these conditions, but it 
is possible to get a qualitative idea of the way the 
noise sets in sharply with the incidence of cavitation, 
rises to a maximum, and then falls off as the cavi- 
tation builds up. 

To produce cavitation easily, a model was used in 
which the nose was flat. This was produced by cut- 
ting off a hemispherical nose at something like three- 
fourths of its radius. Under these conditions, cavi- 
tation set in at A = 2.6. Figure 3 gives an idealized 
curve for this case. As the velocity of the water is in- 
creased, or the pressure is reduced, so that K passes 
through the value 2.5, the sound intensity increases 
sharply by over 20 db. It then continues to rise more 
slowly as K is decreased but finally reaches a maxi- 
mum. 

Because of the difficulties indicated above, there 
may be a good deal of question as to the validity of 
detailed conclusions from this type of work. Never- 
theless, several points stand out. It is most striking 
that the increase in noise level is coincident with the 
onset of cavitation. In other curves given in the re- 
port, the increase is not so sharp and it may be that 
the sharpness of this rise is associated with the shape 
of the body. Nevertheless, this principal conclusion 
is clear that cavitation produces noise, and that the 


CAVITATION NOISE IN TORPEDOES 


15 


noise sets in rather sharply as the cavitation be- 
gins. 

Another point indicated in Figure 3 is that further 
decrease of the cavitation parameter is associated 
with a decrease in the intensity of the noise rather 

V VELOCITY IN FT/ SEC AT 15 FT DEPTH 



1.0 2.0 3.0 


CAVITATION PARAMETER 

Figure 3. Idealized curve of cavitation noise as a 
function of the cavitation parameter for the case of a 
flat-nosed projectile. The sound levels are in decibels 
above the background. 

than a continued building up. This seems to be ob- 
served quite generally in work with models and in 
other experiments designed to study the noise due 
to cavitation. Various suggestions have been made 
to explain it, but none of them are as yet supported 
by adequate experimental evidence. These sugges- 
tions include (1) the idea that the increasing cavita- 
tion tends to absorb the noise and prevent its reach- 
ing the hydrophone; (2) the idea that the noise is 
produced against the surface of the body itself rather 
than throughout the entire volume of the cavitation; 
and (3) the idea that the noise is produced down- 
stream where the bubbles collapse and that this dis- 


tance is greater for highly developed cavitation than 
at the beginning. 

Some indications from the work in the High Speed 
Water Tunnel point to the possibility that the noise 
is not a function of the cavitation parameter alone, 
but of the velocity and the pressure separately. The 
evidence on the point, however, is so uncertain that 
it seems best for the present to regard the cavitation 
noise as a single valued function of the cavitation 
parameter. 



Figure 4. Sound levels in the propeller tunnel of the 
David Taylor Model Basin showing the effect of a 
small nick in the propeller. 

Figure 4 represents some observations from the 
propeller tunnel at DTMB. For this work the pres- 
sure was kept constant and the speed of the water 
past the propeller was varied at the same time that 
the propeller rpm was varied. The hydrophone in 
the water tunnel showed a distinct rise in noise level 
at a fairly definite velocity, and this was approxi- 
mately coincident with the beginning of cavitation. 

The difference between a good propeller and a pro- 
peller having a nick in the blade is made very clear 
in this figure. Cavitation could be observed at the 
blade defect at a rather low speed and when it was 
observed, the noise was very loud. 

In the work carried on by HUSL, noise measure- 
ments could be made only at very low speeds but, 
under these circumstances, the noise was observed to 
increase very rapidly just before tip cavitation could 
be seen. 

4.4 CAVITATION NOISE IN TORPEDOES 

A good illustration of the character of the cavita- 
tion noise is given by a series of observations made 
on the ExF42 mine at DTMB. In this work the mine 
was attached to the strut supporting it from the 


16 


CAVITATION AND CAVITATION NOISE 


high-speed carriage. The motor in the mine was then 
driven at such a speed as to furnish the power neces- 
sary for the propulsion of the mine, while the carriage 
itself provided the power to drag the strut through 
the water. A hydrophone was placed in the towing 
channel so that the mine would pass over it at a dis- 
tance of a few feet. The maximum reading of the 
sound intensity at the hydrophone was then taken 
as a measure of the noise produced by the mine and 
its propeller. The noise increased very little up to a 
speed of almost 15 knots. But at this speed it rose 
almost discontinuously through a considerable range. 
This was observed with the mine submerged 4 ft 
under the surface of the water so that the cavitation 
parameter is about 3.8. 

A principal characteristic of cavitation and the 
noise associated with it is its combined dependence 
on pressure and on speed. If it can be established 
that the cavitation on a given object sets in at a cer- 
tain value of the cavitation parameter, it can be 
readily determined what speed corresponds to this 
cavitation parameter for any given depth of sub- 
mergence. Figure 5 shows the relationship between 



Figure 5. Speed in knots at various depths corre- 
sponding to several values of the cavitation parameter. 


the cavitation speed and the depth of submergence 
in feet for three values of the cavitation parameter. 
Since it was found that the ExF42 mine began to 
cavitate at 15 knots when 4 ft deep, it follows from 
the lower of the three curves that at 120 ft deep, it 
could be driven at over 30 knots before cavitation 
sets in. The other two curves in the figure represent 
other possible situations in which the critical cavi- 
tation parameter is somewhat lower than that for 
the ExF42 mine. It must be remembered, of course, 
that this conclusion is valid only in case the speed of 


all parts of the torpedo are proportionally increased. 
It is assumed that the propeller turns twice as fast 
to give 30 knots as to give 15 knots. This is of course 
not strictly true, since the propeller efficiency de- 
pends on the speed. 

Cavitation noise can be identified by the way in 
which it varies with depth. If cavitation noise is 
present, the noise level will decrease at greater 
depths. In fact, if the principal noise of a torpedo is 
due to cavitation, this noise can always be eliminated 
by operating at sufficient depth. 

The procedure for analyzing torpedo self noise, 
then, consists in making a series of runs at different 
speeds and at different depths. Changes with depth 
at a constant speed can be attributed to changes in 
the noise caused by cavitation, while changes with 
speed at constant depth must be attributed to 
changes in both cavitation and machinery noise. 
Presumably, machinery noise is independent of 
depth, so that if a series of runs can be made at the 
same value of the cavitation parameter, any ob- 
served variation in noise level with speed can be 
attributed to the machinery noise. 

In the process of study leading to the development 
of acoustic torpedoes various measurements have 
been made that show the presence of cavitation 
noise. A set of measurements made by the Bell Tele- 
phone Laboratories [BTL] in the process of studying 
the ExS13 mine is given in Table 1. The mine was 
operated at speeds of 12, 16, and 20 knots and at 



■60 

0 2 4 6 8 10 12 14 

CAVITATION PARAMETER K 


Figure 6. Self noise of the ExS13 mine plotted as a 
function of the cavitation parameter. 

depths ranging from 10 to 80 ft. The data indicated 
are plotted against the cavitation parameter in Fig- 
ure 6, where they fall on a smooth curve within limits 


CAVITATION NOISE IN TORPEDOES 


17 


Table l. 7 Measurements of the self noise of an ExSl3 
mine in a band near 24 kc. Level measurements are in 
dbs above 1 dyne per sq cm. 


SPEED 
knots ft per sec 

Depth 

feet 

Cavitation 
parameter K 

Noise level 
db 

12 

20.3 

10 

6.58 

- 54.0 

12 

20.3* 

52 

13.2 

- 57.0 

16 

27.0 

16 

4.30 

- 46.0 

16 

27.0 

52 

7.46 

- 49.0 

20 

33.8 

10 

2.41 

- 39.4 

20 

33.8 

25 

3.25 

- 42.4 

20 

33.8 

50 

4.65 

- 47.0 

20 

33.8 

80 

6.33 

- 50.5 


of experimental error. Since these measurements 
were made over an extended period of time, and 
since the machinery noise may change from one run 
to the next, some variations due to unknown causes 
are to be expected. In spite of these, the evidence 
points to the conclusion that at depths less than 
50 ft the ExS13 mine, running at 20 knots, was pro- 
ducing essentially cavitation noise. 

Numerous other examples of measurements that 
show the presence of cavitation noise are given in 
Chapter 10. 


Chapter 5 

MACHINERY AND OTHER NOISE 


I n addition to cavitation, the machinery in a 
torpedo is an important source of noise. At first 
thought it might be expected that the machinery 
would be the principal source of noise, for a torpedo 
running out of the water sounds like a tractor. In 
fact, in high-speed torpedoes the engine and gear 
noise may dominate the cavitation, but it seems that 
for torpedoes operating between 20 and 30 knots the 
cavitation and the machinery noise are of nearly 
the same order of magnitude. This has made the 
problem of identifying the source of the noise espe- 
cially difficult. Cavitation noise can be reduced by 
going to greater depths, but if the cavitation and the 
machinery noise are approximately equal in magni- 
tude, the reduction of one does not greatly reduce 
the total. 

The only way to identify machinery noise is to 
eliminate it. This requires a slow process of trying 
one thing after another until some method of sound 
isolation is found that significantly reduces the total 
noise; and this must be done after it is fairly certain 
that the cavitation noise has been effectively elimi- 
nated by going to an adequate depth. Extensive 
studies of this kind have been made by the Harvard 
Underwater Sound Laboratory [HUSL] and some 
studies of motor isolation have been made by the 
Bell Telephone Laboratories [BTL]. This work gives 
general indications as to some aspects of the ma- 
chinery noise but it is far from giving complete in- 
formation on the subject and much important work 
remains to be done. 

5.1 GEAR NOISE 

Meshing gears are a troublesome source of noise 
in all torpedoes containing them. In the standard 
torpedo with counter-rotating propellers the main 
reversing gears seem to be possibly the dominant 
source of noise. Of the weapons with a single pro- 
peller, the Ex20F contains no important gear trains, 
but in the ExFF3 and the ExF42 mine the rudders 
are driven by high-speed motors through a train of 
reducing gears. In these cases there are indications 
from the noise measurements that the steering 
motors and gears fix the background level at about 
— 57 dbs in the neighborhood of 24 kc. This low 
value is only attained by careful selection of the 
equipment, and the levels will frequently be much 
higher when an acoustic test is not used as a basis 
for gear inspection. 


Extensive studies of the main reversing gear prob- 
lem have been made by HUSL 8 and show clearly 
that much noise is due to this source. They investi- 
gated various gear forms and various gear materials 
and were able to produce some reduction in noise. 
The most effective procedure however, appeared to 
be an isolation of the gear system from the shell. 
Two effective methods seemed to be (1) an isolated 
gear housing and (2) an isolated idler gear. 

The gear housing encloses the gears and keeps 
them away from the water. The housing is then 
isolated from the shell by a thick layer of Fairprene. 

The isolated idler gear is a much simpler arrange- 
ment in which the idler gear shaft is not mounted 
directly on the shell but is isolated from it by a thick 
block of Fairprene. Figure 1 shows schematically 
how this is done, and shows the simplicity of such a 
modification for the Mark 18 torpedo. 

The work on these gears has been done principally 
on the Mark 18 torpedo. This appeared to be a par- 
ticularly promising object of study for the following 
reasons. 

1. It was at first expected that an electric motor 
would be a quieter mode of propulsion than a steam 
turbine but the measurements mentioned in Sec- 
tion 3.2 showed this not to be the case. It might be 
suspected on this account that the gears were dom- 
inating at least the machinery noise. 

2. The gears in the Mark 18 are open to the sea 
water and fairly independent of the balance of the 
power plant. This makes their modification a rela- 
tively simple problem and permits a study of the 
wide variety of suggestions necessary in this kind 
of work. 

The measurements made by HUSL indicated that 
at a depth of 50 ft, where presumably the cavitation 
was suppressed and the gear noise was dominant, the 
isolated idler reduced the effective noise level at 
25 kc from —29 to about —44 dbs. This is a very 
significant reduction and makes the Mark 18, at this 
depth, suitable for acoustic control. 

Since the reversing gears appear to be such an im- 
portant source of noise one might think it better to 
use only a single propeller and no gears. However, 
for torpedoes running at speeds much above 20 knots 
it is almost essential to use two counter-rotating 
propellers. This is true for two reasons. 

1. It is very difficult to balance the torque of a 
single propeller because a torpedo is essentially 


18 


GEAR NOISE 


19 



LOOKING FORWARD 

Figure 1. A method of idler gear isolation for the Mark 18 torpedo. 


20 


MACHINERY AND OTHER NOISE 


cylindrical. It is possible to put some of the elements 
off center to create a stabilizing moment, but there 
is a limit to the amount of moment that can be cre- 
ated this way. In addition, when the torque is bal- 
anced in this way the heel of the torpedo is critically 
dependent on the power output and changes if the 
motor slows down. 

The torque of a single propeller is given by 
L = 5,250 hp/rpm 

so that for 180 hp at 1,600 rpm the torque is 592 ft-lb. 
This is not easy to balance by shifting weights even if 
the torpedo is allowed to heel as much as 45 degrees. 

The torque can also be balanced by skewing the 
fins, and possibly the rudders and elevators also, to a 
proper angle and the necessary angle will be roughly 
independent of the speed. This method, however, 
introduces some additional drag. 

In addition to the torque necessary for steady run- 
ning, the initial acceleration of the propeller tends to 
turn the torpedo past its equilibrium position and 
may well turn it over. Problems of this kind can be 
handled in a low-power, low-speed torpedo but they 
become more difficult as the speed is increased. Pos- 
sibly a practical limit for single propeller torpedoes 
lies somewhere between 20 and 25 knots. 

2. The load per unit area on a single propeller 
must be almost twice as great as on a pair of pro- 
pellers. Although only entirely inadequate informa- 
tion is available on the properties and characteristics 
of counter-rotating propellers, it seems probable that 
cavitation would be more easily avoided when two 
propellers are used than when all of the thrust is de- 
veloped by a single propeller. 

Since two propellers seem to be highly desirable, 
if not almost essential, it is necessary to isolate the 
gears carefully or else to drive the propellers sepa- 
rately. The normal torpedo drive has been based on 
the idea that both propellers should turn at the same 
speed. This means that the propellers must be care- 
fully balanced so that they apply the same torque. 
To the extent that they are balanced the resultant 
torque is zero and the torpedo will travel on an 
even keel. 

For turbine-driven torpedoes the gear system is 
necessary for speed reduction as well as to provide 
the two directions of rotation. In case of electric 
torpedoes a high-speed motor may be enough lighter 
than a low-speed motor so that its use, together with 
the reducing gears, is advantageous. If these systems 
are used, some kind of gear system is necessary. 


However, it is possible in electric torpedoes to 
drive each propeller separately. This was done by the 
Westinghouse Electric Company in their Mark 26 
torpedo and also in their first model of an ExS29. 
These motors were mounted one in front of the other 
and the shaft of the forward one passed through the 
hollow shaft of the after motor to the after propeller. 
The after motor drove the forward propeller through 
a hollow shaft. 

A possibly more satisfactory arrangement is the 
use of a counter-rotating motor. In this the field coils 
turn in one direction and the armature in the other. 
The torque is applied between the two rotating parts 
of the motor, each of which is connected to one pro- 
peller. This system has a number of advantages. 

1. There is no torque to compensate, either during 
running or during starting, so a small metacentric 
height is adequate to insure stability and to prevent 
heel. No propeller balancing is needed. 

2. There are no gears and consequently there is 
no gear noise. 

3. The relative speed of rotation of the two parts 
of the motor is twice the speed of rotation of one 
propeller. This makes possible a high-speed, lighter 
weight motor without gears. Motors of this type 
have been built by Stone & Co. of England and by 
the Electrical Engineering and Manufacturing Cor- 
poration of Los Angeles. Tests have shown their 
advantages with respect to torque balancing. 

As a conclusion it may be repeated that for a quiet 
torpedo it is necessary either to eliminate the re- 
versing gears or to isolate them. Either can be done. 

5.2 OTHER MACHINERY NOISE 

There are numerous other possibilities of inter- 
ference from the mechanism within a torpedo. The 
control apparatus is often a source of noise. As al- 
ready mentioned, if the rudders are driven by elec- 
tric motors through a gear train, this system may set 
the lower limit to the self-noise level. In the case of 
pneumatically operated controls it is probable that 
noise is also produced but no observations are avail- 
able that clearly indicate the significance of this 
source. 

The reciprocating engines of the British torpedoes 
produce vibrations of frequencies corresponding to 
the number of revolutions per second and the har- 
monics of this frequency. The frequency with which 
the propeller blades pass the rudders may also appear 
in the noise spectrum of a torpedo. 


OTHER SOURCES OF NOISE 


21 


Apparently the commutator and brushes of a d-c 
motor produce a significant noise. This is apparently 
transmitted to the hydrophones by way of the shell. 
Either it is transmitted through the shell directly or 
is radiated by the shell into the water and then is 
picked up by the hydrophones. Probably neither 
statement is an adequate description of the interre- 
lated action of the shell and the surrounding water 
in transmitting this noise, but in any case isolation 
of the motor from the shell seems to reduce its effect. 

In the process of studying the isolation of the 
motor from the shell, BTL made a series of observa- 
tions that led to the conclusion that some types of 
isolating material were quite effective in inhibiting 
the transmission of shear vibration but were rela- 
tively ineffective against compressional waves. In 
such a case it was possible to construct a type of 
mounting in which two isolating layers were provided 
at right angles to each other. Figure 2 shows sche- 
matically the idea involved. 



Figure 2. A schematic illustration of a method of 
using a material that isolates against shear vibrations 
only. 


In general it seems that machinery noise has rather 
well-defined frequencies in the low-frequency region, 
below 5,000 c, but that at high frequencies the vari- 
ous harmonics are so close together that it is not sig- 
nificant to speak of individual frequencies. At lower 
frequencies it might be possible to place a sharply 
tuned hydrophone at a frequency between the char- 
acteristic frequencies of the machinery noise. It 
would be very difficult, however, to specify a manu- 
facturing procedure that would guarantee satisfac- 


tory discrimination in all cases. The general pro- 
cedure thus far has been to use a moderately high 
frequency where the exact frequency is of less im- 
portance and where the variation in effective noise 
level from one torpedo to the next can be kept 
within reasonable limits. 

5.3 NOISE DUE TO GAS FLOW 

Very probably the passage of high-pressure air 
through restrictions and valves produces a certain 
amount of high-frequency noise that can be trouble- 
some. This can possibly be controlled by isolating 
the pipes, and fittings from the shell carrying the 
hydrophones. In general it seems that very little 
noise of this type is carried through the air but it is 
very effectively transmitted by metallic contact. 

The emission of gas into the turbulent boundary 
layer surrounding a torpedo may well produce a good 
deal of high-frequency noise. Experiments in which 
an ExF42 mine was propelled by a sea water battery 
from which gas was exhausted through an opening in 
the top showed a noise level over 15 db above the 
normal running noise. This was almost as much as 
produced by propeller cavitation. 

5.4 OTHER SOURCES OF NOISE 

Although noise due to cavitation and noise asso- 
ciated with the machinery constitute the principal 
types of torpedo noise, there are some other types 
that may be of importance under certain circum- 
stances. These have been the object of only a limited 
amount of study, and the following comments are 
essentially indications of subjects for study. 

5.4.1 Water-Flow Noise 

In some cases it appears that the turbulent surface 
layer of water flowing over a hydrophone produces 
an effect that is principally noticeable in the lower 
frequencies, below 3,000 c. This seems to be analo- 
gous to the phenomenon of “windage” in which a 
wind blowing over a microphone may produce a con- 
siderable response, although the noise radiated into 
the air is negligible. 

Water-flow noise produced in this way would not 
be picked up in an external hydrophone but it might 
well contribute to the response of a hydrophone 
mounted in the body and must be considered as a 
possible source of self noise. Presumably it can be 
avoided by shielding the hydrophone itself from 


22 


MACHINERY AND OTHER NOISE 


direct contact with the moving water, or possibly by 
placing it near the nose where the turbulent surface 
layer has not had a chance to develop. 

Some observations have been made on this type of 
noise by BTL 9 but it does not seem to be of signifi- 
cance above the audible range of frequencies. 

In some cases it appears that the impact of the 
water on parts of a torpedo may set up resonant vi- 
brations. If a stream of water is incident on a tuning 
fork held under water, the fork is set into vibration 
and the sound is radiated through the water. Nor- 
mally the resonant frequencies that are not too highly 
damped are in the acoustic range and not much above 
it. 

Both of these kinds of water-flow noise need more 
study before appraisal of their significance is possible. 
However, on the basis of preliminary indications, it 


seems that the higher the frequency the less they are 
of importance. 

5.4.2 Propeller Vibration 

During some of the studies of experimental bodies 
of the type of the ExF42 a rather loud whine of about 
2,000 c was observed. Studies of the propeller showed 
a natural frequency of vibration near 2,200 c which 
changed to 2,000 when the propeller was immersed 
in water. Apparently this vibration was excited 
either by the motor through the propeller shaft, or 
by the water forces. Various methods were tried by 
BTL 10 for reducing this vibration and it can prob- 
ably be controlled. However, again it is fortunate 
that vibrations of this kind will probably be confined 
to the region of acoustic frequencies and can be 
avoided by working in the supersonic region. 


Chapter 6 

HYDROPHONE DISCRIMINATION AND ISOLATION 


I n constructing an acoustically controlled tor- 
pedo the effective noise level can be reduced just 
as well by reducing the response of the hydrophone 
to the background and self noise as by reducing the 
level of the noise itself. This must be done of course, 
without reducing the hydrophone response to the 
desired signal. For this reason it is convenient and 
customary to express the background and self-noise 
levels in terms of the intensity of the plane wave in- 
cident on the hydrophone from the direction of maxi- 
mum sensitivity that produces the same rms re- 
sponse as does the noise. 

When it is stated that the self-noise level is —20 
dbs, the meaning is that the electrical response of 
the hydrophone and its circuit has the same rms 
value as though a plane wave of intensity level — 20 
dbs were incident on the hydrophone from the direc- 
tion of maximum response. The cause of the response 
need not be sound, in the simplest sense, at all. 

The self noise may be electric interference picked 
up by the circuit because of inadequate shielding. In 
many cases a large amount of the self noise appears 
as vibrations in the torpedo shell which are trans- 
mitted directly to the hydrophone so as to set it in 
vibration. In all cases it is equivalent to a noise and 
must be controlled for effective operation. 

6.1 DISCRIMINATION AGAINST WATER 
BACKGROUND NOISE 

Water background noise usually comes more or 
less uniformly from all directions, i.e., it is isotropic. 
A nondirectional hydrophone responds to the total 
sound intensity, regardless of direction. A directional 
hydrophone, on the other hand, responds only to 
sounds coming from certain directions and therefore 
responds less to an isotropic sound field than does the 
nondirectional hydrophone. This difference in re- 
sponse is described by the directivity index. 11 

If a hydrophone responds uniformly to sound in- 
cident within a solid angle ft, and has a zero response 
outside of this angle, then its response to an isotropic 
sound field will be (ft/ of that of nondirectional 
hydrophone. Its directivity index will then be 



and the response to an isotropic sound field whose 
level is L will be ( L + D). For purposes of reducing 


the response to the water background noise it is de- 
sirable to have D as much negative as possible, i.e. 
to have the hydrophone sensitive in as small a solid 
angle as possible. Since, however, the water back- 
ground is the limiting factor only in rather special 
cases, this is not a dominant factor. 

The directional patterns of the ExF42 crystal hy- 
drophone and the HUSL twelve-tube magneto- 
striction hydrophone are shown in Figures 1 and 2. 



90 110 130 150 170 -170 -150 -130 -110 -90 -70 -50 -30 -10 <10 30 50 70 90 

ANGLE IN DEGREES 


Figure 1. Directional response pattern of a crystal 
hydrophone as used in the ExF42 mine. 12 



-160 -120 - 80 - 40 0 40 80 120 160 


ANGLE WITH TORPEDO AXIS IN DEGREES 

Figure 2. Directional response pattern of a 12-tube 
magnetostriction hydrophone. 13 

These patterns are not entirely independent of the 
hydrophone mounting, and the curves shown repre- 
sent the hydrophone mounted in a portion of a tor- 
pedo body. The two directive indexes, —11.0 and 
— 13.6 db, show that for an average background level 
of — 54 dbs at 25 kc the hydrophone responses will be 
about —65 and —68 dbs respectively. This is low 
enough so that it rarely compares wdth the other 
noises present. 

For a given hydrophone and mounting, the direc- 
tivity index becomes more negative as the frequency 
increases. This merely means that a given hydro- 
phone is more directional for short wavelengths than 
for long. Since, in addition, the water noise level in 
general decreases at higher frequencies, the response 




23 


24 


HYDROPHONE DISCRIMINATION AND ISOLATION 


of a hydrophone to background noise falls off quite 
rapidly as the frequency increases. 

6.2 DISCRIMINATION AGAINST 
CAVITATION 

In most cases the principal cavitation around a 
torpedo is associated with the propeller. The noise 
from this cavitation is then transmitted partly 
through the water and partly through the torpedo 
shell to the hydrophone. Since the sound travels al- 
most parallel to the shell, the distinction between 
these two modes of transmission is not sharp, but it 
indicates roughly two ways of making the hydro- 
phone discriminate against the propeller noise. 

The hydrophone whose directional pattern is 
shown in Figure 1 was mounted in the side of a cylin- 
drical body. The response to the fear along the 
cylinder is about 27 db below that at right angles to 
the cylinder. To the extent, then, that a directional 
pattern of this kind really represents the response of 
the hydrophone to the cavitation noise around the 
propeller, one may say that this hydrophone dis- 
criminates against cavitation noise to the extent of 
27 db. In the ExF42 mine this discrimination is ap- 
parently of little importance, since there is practi- 
cally no propeller cavitation noise. 

Figure 3 shows a pattern for the same type of 



-90 - 70 -50 -30 -K) K> 30 50 70 90 110 130 150 170 -170 -150 -130 -110 

ANGLE WITH TORPEDO AXIS IN DEGREES 

Figure 3. Directional response pattern of a crystal 
hydrophone, of the type used in the ExF42 mine, 
placed in a hemispherical nose. 

hydrophone placed in the hemispherical nose of the 
torpedo. In this case it was 70 degrees off the axis. 
The response at 180 degrees, or straight back, is pos- 
sibly 32 db below the maximum or some 5 db better 
than when mounted in the side position. 

HUSL has made an important series of measure- 


ments of the response to noise sources of hydrophones 
mounted in the head of a Mark 18 torpedo. The head 
was suspended under water and the sound source was 
similarly immersed a short distance away. The sig- 
nificant conclusion is that the discrimination is a 
function not only of the hydrophone and its immedi- 
ate mounting but also of the nature of the shell in 
which it is mounted. This merely indicates the com- 
plication of the problem and shows that a simple 
calculation of the directional pattern is not adequate 
for describing the discrimination of the hydrophones 
against cavitation noise. 

The assumption that the cavitation noise can be 
regarded as located in the water outside the torpedo 
is probably also inadequate. There is certainly an 
absorption of sound by the afterbody and possibly 
some of the sound is produced in contact with the 
propellers or other parts of the torpedo. 

In particular it may be that some of the noise as- 
sociated with the reversing gears is really cavitation 
noise. The water in the gears is certainly subject to 
variations in pressure that might well produce cavi- 
tation. This cavitation is in intimate contact with 
the gears and the noise may well be transmitted 
through the gears themselves. Isolation of the gears 
would then be an effective way of shielding the hy- 
drophones from this part of the cavitation noise as 
well as from what might be more conventionally 
called machinery noise. 

6.3 DISCRIMINATION AGAINST 
MACHINERY NOISE 

Machinery noise is produced in direct metallic 
contact with the shell and is transmitted both 
through the shell and the water to the hydrophones. 
Apparently the exact process is a complicated inter- 
action between the water and the shell that can be 
only partly described by saying that some energy is 
radiated into the water by the afterbody shell and 
that the energy is then absorbed from the water by 
the hydrophone. Nevertheless this partial description 
indicates why an isolation of the hydrophone from 
the shell is only partly effective. The discrimination 
seems to be improved by breaking the shell at one or 
more points, and also by a layer of absorbing ma- 
terial on the inside of the shell. 




Chapter 7 

TOTAL TORPEDO NOISE 


T he previous chapters have described the var- 
ious sources of self noise in a torpedo and have 
indicated the nature of the evidence for the existence 
of cavitation noise and machinery noise that can be 
separated from each other to some extent by meas- 
uring the noise at various depths of submergence. A 
complete study of the noise of any one torpedo would 
involve running it at a variety of depths and speeds. 
From the curves of noise level as a function of depth 
for a given speed the curve of cavitation noise could 
be determined, and from a curve of noise as a func- 
tion of speed for a constant cavitation coefficient, 
the machinery noise as a function of speed could be 
determined. By doing the same thing with different 
hydrophones, and different hydrophone positions, 
at least the relative values of the hydrophone dis- 
crimination could be determined. 

Such a complete series of measurements is not 
available for any torpedo. The noise measurements 
that have been made have been carried out under the 
press of wartime conditions and the necessity of pro- 
viding quick and rough information for the construc- 
tion of usable weapons. These measurements do, 
however, provide some crude indications of the 
values of the various quantities. By using a liberal 
quantity of skepticism regarding the accuracy of the 
measurements and a certain amount of imagination, 
it is possible to build up curves that illustrate many 
of the trends shown in the observations. 

With this object in mind, in this chapter, a series 
of assumptions will be made on the basis of which a 
group of theoretical curves can be plotted. These 
curves will not agree in any great detail with the 
observations, but they will illustrate the types of 
curves that may be met and the ways in which the 
different noise sources make themselves apparent. 

7.1 ASSUMPTIONS 

7.1.1 Water Background Noise 

In general there is a small background noise due 
to sources outside the torpedo. This is roughly iso- 
tropic and its full value will be measured by a non- 
directional hydrophone. In the neighborhood of 25 kc 
it will be assumed that the level of this noise is 
— 54 dbs with one dyne per sq cm as the reference 
level. 


7.1.2 Cavitation Noise 

It will be assumed that for normal torpedoes the 
curve of cavitation noise level as a function of cavi- 
tation parameter is that given in Figure 1. The de- 

55.1 39.2 32.0 24.4 19.5 16.3 

VELOCITY l/~ AT 15 FT SUBMERGENCE 


72.8 51.5 42.0 36.4 32.6 29.7 

VELOCITY V AT 50 FT SUBMERGENCE 



Figure 1 . Assumed form of cavitation noise as a func- 
tion of the cavitation parameter K. 


tailed form of this curve is not very significant. The 
principal feature is that it rises rapidly as the cavi- 
tation parameter decreases, and then levels off. 

The curve as drawn in Figure 1 is based on the use 
of the forward speed of the torpedo in evaluating the 
cavitation parameter. If the cavitation were on the 
nose, or on some fixed part of the torpedo, such as a 
fin or a rudder, this would unquestionably be the 
correct velocity. Since, however, the cavitation prob- 
ably develops on the propellers, the forward velocity 
is the only correct one to use when torpedoes with 
approximately similar propellers are being com- 
pared. Apparently most normal torpedoes are suffi- 
ciently similar so that the forward speed is propor- 
tional to the propeller tip speed in most cases. How- 


S EC RET 


25 


26 


TOTAL TORPEDO NOISE 


ever, when single propellers are used, or propellers of 
radically different pitch, it is necessary to use a cor- 
rected parameter K' . If v p ° is the propeller tip speed 
of the normal torpedo at the running speed in ques- 
tion, and if v p f is the tip speed of the modified type 
of propeller, then the value of the cavitation param- 
eter to be used with the modified propeller is 

With this understanding the velocity that is effective 
in determining the cavitation parameter is propor- 
tional to the propeller tip speed. 



SPEEO IN KNOTS 


Figure 2. Assumed cavitation noise level as a func- 
tion of speed for three depths and the assumed machin- 
ery noise level. 

7.1.3 Machinery Noise 

It will be assumed that the machinery noise is pro- 
portional to the sixth power of the torpedo speed. 
This seems a rather surprising law but it agrees with 
the general trend of the observations moderately 
well. 

A British report 14 describes measurements of tor- 
pedo noise in which the sound intensity was pro- 
portional to the eighth power of the speed. However, 
since the measured noise probably included cavitation 
noise, the observation is not definitive. 

It might seem reasonable that the noise would 
vary as the third power of the speed, since the power 


of the engine follows this law and it might be ex- 
pected that a constant fraction of the power would 
go into noise. Nevertheless it appears that the v 6 
law agrees with the overall picture of the observa- 
tions as well as anything else, and it will be used in 
the following curves. Let it be emphasized again, 
however, that this particular form of law has no 
direct experimental basis. 

7.1.4 Hydrophone Discrimination 

Each hydrophone, hydrophone position or hydro- 
phone isolation will be represented by a certain sensi- 
tivity to, or discrimination against, cavitation noise 
on the one hand and machinery noise on the other. 
In addition there will be a certain discrimination 
against background noise which is given by the di- 
rectivity index of the hydrophone. This latter, how- 
ever, is of negligible importance in most cases. 



O 10 20 30 4 0 50 60 

FORWARO SPEED IN KNOTS 


Figure 3. Total noise at 15-ft depth on the basis of 
idealized assumptions. Points are representative of 
the observations shown in Figure 1 (Chapter 1) and are 
the same as those in Figure 2 (Chapter 1). 

Externally Measured Noise 

As was indicated in Chapter 3 the noise from most 
standard torpedoes can be represented as a function 
of speed by a single curve. The indications seem to 





ASSUMPTIONS 


27 


be that between 20 and 30 knots cavitation noise 
and machinery noise are near to the same order of 
magnitude but that cavitation noise is definitely 
predominant at 30 knots. For speeds as high as 45 



0 10 20 30 40 50 60 

FORWARD SPEED IN KNOTS 

Figure 4. Total external torpedo noise at 40-ft depth 

on the basis of idealized assumptions. 

knots the machinery noise seems to have increased 
again until it is in the lead. We may then construct 
curves on the following basis. 

1. Background noise level is —54 dbs. 

2. Cavitation noise level is given in Figure 1. 

3. Machinery noise level is given by L(v) = 60 
log v — 120, where v is the speed in feet per second. 

Figure 2 shows the cavitation level and the ma- 
chinery level plotted separately and Figure 3 shows 
the sum. These figures show the way in which first 
one source of noise and then the other can be dom- 
inant. The five points indicated are the points from 
Figure 2 in Chapter 3 and indicate that in a very 
rough way the composite curve of noise as plotted is 
in agreement with the observations. 

Figures 4 and 5 show a way in which the noise- 
speed curves may depend upon the depth at which 
they are taken, and Figure 6 shows the correspond- 


ing way in which the noise may depend on depth at 
a constant speed. Characteristic of the latter curves 
is the drop with increasing depth to the machinery 
noise limit beyond which the noise does not decrease. 



FORWARD SPEED IN KNOTS 


Figure 5. Total external torpedo noise at 60-ft depth 
on the basis of idealized assumptions. 

Self-Noise Measurements 

During the past two years the Harvard Under- 
water Sound Laboratory [HUSL] and the Bell Tele- 
phone Laboratories [BTL] have made extensive 



Figure 6. Total external torpedo noise as a function 
of depth for three different speeds, based on idealized 
assumptions. 


28 


TOTAL TORPEDO NOISE 


measurements of self noise in various torpedoes. 
Neither of them has made as complete a set as might 
be desired but it is possible to use their observations 
for crude estimates of the quantities involved and as 
a rough justification for the description of this noise 
that has been used. 

In trying to understand self-noise measurements 
the discrimination of the hydrophones must be taken 
into account, and it is in this respect that self-noise 
observations will differ from the external measure- 
ments described above. 



10 15 20 25 30 35 40 

SPEED IN KNOTS 


Figure 7. Self-noise level at 25 kc for two different 
depths. The hydrophone is assumed to have a dis- 
crimination of 20 db against cavitation noise and 15 
db against the machinery noise. The points are taken 
from measurements by the Harvard Underwater Sound 
Laboratory. 16 

1. Figure 7 shows the result of assuming a 20-db 
discrimination of the hydrophone against both cavi- 
tation and machinery noise. This means that the 
response is 20 db below that of a nondirectional 
hydrophone 6 meters from the torpedo. The points 
shown are taken from a Harvard report 15 of measure- 
ments on a Mark 18 torpedo. Although nothing like 
quantitative agreement of the points with the curves 
is claimed or is to be expected, it seems clear that the 
points show that the increased depth has reduced the 
cavitation noise. 

2. Another illustration of self-noise measurements 
is given in Figure 8. This shows the noise as a func- 
tion of depth for two different speeds. Since the tor- 
pedo was a Mark 18 driven by a single propeller, it 


was necessary to make use of a K' = 0.455 K instead 
of the usual K. In plotting the theoretical curves it is 
assumed that the hydrophone discrimination against 
the cavitation was 30 db and against the machinery 
noise 18 db. The points are taken from one of the 
Harvard reports. 16 



0 20 40 60 80 100 120 140 

DEPTH IN FEET 

Figure 8. Self-noise level at 25 kc for two different 
speeds. The cavitation is described in terms of a pa- 
rameter K' = 0.455A, and the hydrophone discrimi- 
nation was assumed to be 30 db against cavitation noise 
and 18 db against machinery noise. The points rep- 
resent measurements by the Harvard Underwater Sound 
Laboratory. 16 

3. Figure 9 shows that it is possible to understand 
a curious phenomenon noted in some of the measure- 
ments in which nose hydrophones were compared 
with body hydrophones. The Harvard observations 17 
seemed to show that in the study of the Mark 18 
with a single propeller the curves of noise level as a 
function of depth for the body hydrophone seemed 
to cross the corresponding curve for the nose hydro- 
phones. Although it is important not to stress too 
much the accuracy of a few observations under diffi- 
cult conditions, it is of interest to note that this effect 
can be produced if the different hydrophones are 


■ i 


ASSUMPTIONS 


29 


assigned the proper discrimination. The fact that the 
change in hydrophone position affects the two types 
of discrimination differently makes possible such 
phenomena, and it is clear that it is quite reasonable 
to expect such differences. 

4. Figure 10 shows a pair of curves of self -noise 
level as a function of depth corresponding to speeds 



DEPTH IN FEET 

Figure 9. Self-noise level at 25 kc for two different 
speeds and two different hj'drophones. In addition to 
the idealized assumptions as to the magnitude of the 
noise produced, it is assumed that the nose hydrophones 
discriminate against cavitation by 23 db and against 
machinery noise by — 1 db, while the body hydrophones 
discriminate against the cavitation noise by 20 db and 
against the machinery noise by 8 db. The cavitation 
parameter K' = 0.455K is used. 


of 33 knots and 28 knots respectively. These have 
been drawn on the assumption that the hydrophones 
discriminate against the cavitation noise to the extent 
of 25 db and against the machinery noise to the ex- 
tent of 20 db. Points are shown indicating results 
obtained by HUSL on the Mark 13 torpedo. The 
coincidence of the two points at the 50-ft depth is 
of course entirely contrary to the general trend of 
the curves which are approaching limiting values 
corresponding to the difference between the ma- 



Figure 10. Self-noise level at 25 kc for two different 
speeds of the Mark 13 torpedo. The hydrophone ap- 
pears to be such that a 25-db discrimination against 
cavitation noise is assumed in drawing the curves. The 
discrimination against the machinery noise is assumed 
to be 20 db. The points are taken from a report from 
the Harvard Underwater Sound Laboratory. 18 


GO 

O 

z 


UJ 

> 

UJ 


UJ 

</> 


-10 


-20 


-30 


-40 


-50 


-60 


-70 


SPEED IN KNOTS 



Figure 11. Self-noise level at depths of 15 and 50 ft 
as a function of speed. The solid curves correspond to 
a discrimination against cavitation noise of 20 db and 
against machinery noise of 15 db. The dotted curves 
correspond to the same discrimination of 20 db against 
cavitation noise, but a discrimination of 30 db against 
machinery noise. 



30 


TOTAL TORPEDO NOISE 


chinery noise of 28 knots and that of 33 knots. Since, 
however, these curves are separated by a matter of 
only 6 db, a considerable amount of this discrepancy 
could possibly be attributed to experimental diffi- 
culties. 

5. The effect of reducing or discriminating against 
machinery noise is very much dependent upon tor- 
pedo speed and depth. This is brought out clearly in 
Figure 11. In this figure the solid curves are the same 
as those in Figure 7. The dotted curves correspond to 
a discrimination against cavitation noise of 20 db, 
but a discrimination against machinery noise or else 
a reduction in machinery noise of 30 db. It is clear 


from these curves that at 15 knots the machinery 
noise, with only 15-db discrimination against it, was 
the principal contribution to the self noise. At 35 
knots, the cavitation is much more prominent, al- 
though the machinery noise is clearly more signifi- 
cant at 50 ft than at 15 ft. This kind of curve illus- 
trates the way in which a real reduction in machinery 
noise might be completely masked if the measure- 
ments were made under such conditions that the 
cavitation noise dominates. On the other hand, it 
shows that the observed fact of a reduction of 15 db 
in the machinery noise, which is clearly evident at 
15 knots, is no longer nearly so striking at 30 knots. 


Chapter 8 

TRANSFORMATION OF ACOUSTIC SIGNAL INTO A 
DIRECT-CURRENT VOLTAGE 


T o make use of the acoustic signal for control of 
the torpedo, a circuit must be devised that trans- 
forms the hydrophone response into a suitable signal 
for actuating the rudder. There are many ways of 
doing this, but for illustrating the principles involved 
the schematic diagram of Figure 1 may be regarded 



Figure 1 . Schematic diagram of a simple circuit for 
acoustic listening homing. 

as typical. This circuit is drawn for steering in the 
horizontal plane and it is intended to operate so that 
the rudder position is a function of the direction from 
which the acoustic signal is coming. When this direc- 
tion is near the axis of the torpedo, the rudder de- 
flection is intended to be at least roughly propor- 
tional to the angle from which the signal comes. 


8.1 DESCRIPTION OF A SIMPLE 
CIRCUIT 

H p and H s are respectively the port and starboard 
hydrophones. The signal from each of these first goes 
through an amplifier and then to a detector, which 
may be considered to be a “square law” detector. It 
is essential that both amplifiers have the same gain, 
and the maintenance of this equality of gain is one of 
the principal problems in designing a suitable circuit. 
Nevertheless, this difficulty will be ignored for the 
present, and it will be assumed that the gains are 
equal. This problem will be mentioned again later. 
Under this assumption, the d-c outputs of the de- 
tectors will be proportional to the intensities of the 
responses of the corresponding hydrophones. 

The two d-c signals, of the same sign, are then put 
into the condensers C p and C 8 and the resistances R p 
and R s . The potential V d across the resistances is 
then proportional to the difference between the re- 
sponses of the two hydrophones. It is positive when 
H 8 gives the stronger response and negative when H p 
is affected more strongly. The condensers and re- 
sistances are adjusted to give the desired time con- 
stant to the system. When they are large, the signal 
is averaged over a correspondingly long period of 
time and the system is not much affected by sudden 
sharp bursts of noise. On the other hand, the time 
constant must not be long compared with the dy- 
namic time constants of the torpedo body, or the 
system will not respond quickly enough to steer 
correctly. 

The voltage V d is then combined with the voltage 
from the potentiometer P r and applied to a polarized 
relay. The resistance of this relay must be large, since 
it is parallel with R p and R s and affects the time con- 
stant of the circuit. If the resultant voltage V r has 
one sign, the arm is pulled to contact a; if it has the 
other sign, the arm moves to contact b. There may 
also be a neutral position taken if | V r | < F r0 . When 
the relay makes one contact or the other, the battery 
B m drives the motor in one direction or the other. 
This turns the rudder and, since the rudder is me- 
chanically connected to the potentiometer arm, the 




31 




32 


TRANSFORMATION OF ACOUSTIC SIGNAL 


motion of the rudder changes the potential applied 
from the potentiometer P r . If the relay has a neutral 
position, the motor will stop when V r = ( V d — V p ) 
is near zero. This is when the potentiometer voltage 
is equal and opposite to V d . The rudder deflection is 
then proportional to the voltage V d , since it is pro- 
portional to V p . This is of course only true until the 
arm gets to the end of the potentiometer or the rud- 
der reaches its stops. If the relay has no neutral po- 
sition, the rudder will oscillate about the equilibrium 
position with a frequency that depends on the con- 
stants of the motor and the relay, and it will be the 
equilibrium position that is proportional to V d . 

It is clear that the relay will not operate on an in- 
finitesimal voltage, and furthermore that the voltage 
corresponding to full rudder deflection is a finite 
voltage. On the other hand, the acoustic signal varies 
through wide limits as the torpedo approaches its 
target. For this reason, some means are necessary to 
change the gain of the amplifiers in accordance with 
the strength of the signal. One method of doing this 
is to use an automatic volume control governed by 
the potential of the point P. This has a potential 
proportional to the average intensity of the responses 
of the two hydrophones. It is amplified as necessary 
and applied to the grids of the amplifiers in such a 
way that the gain is reduced when the average re- 
sponse increases. It may be designed so that the 
gain G has its maximum value G 0 until the mean re- 
sponse corresponds to some value P 0 , after which 
the gain is inversely proportional to the potential P. 

Each hydrophene has a response that is a function 
of the direction from which the signal comes. Let 
this be R p (6) for the port hydrophone and R s (S) for 
the starboard hydrophone. Let d be the angle that 
the direction of the signal makes with the axis of the 
torpedo. Consider 6 positive when on the starboard 
side and negative on the port side. R p (6) then has its 
maximum for a negative value of 0 and R s (0) has its 
maximum for a positive value of 0. These responses 
R p and R s are expressed in db with reference to 1 volt 
rms output for a sound wave of 1 dyne per sq cm rms 
sound pressure. Correspondingly, let R p ° and R° 
be the maximum responses of the corresponding hy- 
drophones. If the hydrophones and their correspond- 
ing amplifiers are perfectly balanced and matched, 
R a ° = R p ° a,nd R p (0) = R 8 (—d). However, the match- 
ing is not perfect, and especially in setting up a man- 
ufacturing procedure some lack of balance must be 
expected. Since the amount of the unbalance is one 
factor in limiting the performance, let R s ° = R° + 8 


and R p ° — R° — 8. In order not to complicate the 
discussion here too much, it will be assumed, how- 
ever, that the two hydrophone patterns are similar, 
so that 

R P (0) + 8 = R s ( — 0) — 8. 

Let L n be the effective level of the self noise plus the 
background noise at the frequency passed by the hy- 
drophones and amplifiers. As indicated previously 
this means that the self noise produces a response in 
the hydrophones equivalent to that produced by a 
plane wave of level L n incident from the direction of 
maximum response. Then in the absence of a signal, 
the voltages will be 

V p = G10 (Ln + R v 0)/10 

and 

V s = GlO (Ln + R8 ° )/10 . (1) 

Under these circumstances, the potential of the point 
P will be 

P = -lO (Ln + fl ° )/10 (10 5/1 ° + io -5/1 °), (2) 

2 

and the voltage V d will be 

V d = G10 {Ln + RO)/1 ° (10 5/1 ° - IQ -5710 ). (3) 


This V d will be zero only if the two hydrophones with 
their amplifiers and detectors are perfectly balanced, 
i.e., when 8 = 0. In general, this is not the case, and 
a slight differential V d will exist in the absence of any 
signal at all. 

If now a signal of level L s is incident from the 
angle 6, the responses of the two hydrophones will be 


V s = G { 10 (Ln + R ° + 5)710 + 10 {L * + Ra w)/1 ° } 
Vp = G { 10 (Ln + R ° ~ 5)710 + 10 {L 8 + Rp m/1 ° j 


( 4 ) 


Y a — Glut's + R°)/10 | 10 (£n - L s + 5 )/ 10 

_j_ 1 () (ft 8 ( 0 ) _ R 8 ° + 5)/10 J ( 5 ) 

Vp = 6rl0 (Ls + ft ° )710 | 10*» ~ L s - 5 )/ 10 

_j_ iq(«„(«) - Rp° - S)/10J 

V d = G10 {L8 + RO)/1 ° {io (L n- L 3 )/10 (io 5/1 ° - 10- 5/1 °) 

_|_ - R s ° + «/10 _ - R P ° - «)/lO J (g) 

P = - 10 (L « + RO)/10 { 10 (Lw - is)/10 (10 5/1 ° + 10- 5710 ) 
2 

^ 0 (^( 0 ) _ R s ° + «)/10 _|_ 1 O («p(0) - Rp° - «)/io j ( 7 ) 


The quantity P determines the gain of the ampli- 
fiers. For illustration, assume that the A VC is so 
built that P never exceeds the value 2 v. As long as 


DESCRIPTION OF A SIMPLE CIRCUIT 


33 


2 P/G ^ 4, G = 1.00, but for larger values P is held 
constant at 2.00. 

The question of the value of 5 is largely one of 
manufacturing reproductibility. For illustration, 
assume 5=1. This is an extreme value, since the 
experience on the ExF42 mine suggests that 5 ^ 
0.2 can be maintained by careful balancing of the 
system after the hydrophones are installed in the 
body and connected to the circuit. Nevertheless, 


time constants of the circuits are not involved. Fig- 
ure 2 shows the voltage V d when the gain is so ad- 
justed that the self noise gives a voltage of 1 volt 
when the hydrophone response is R°. Since the hy- 
drophones are expected to deviate from this by 1 db, 
the noise, in the absence of any signal, produces a 
response from the starboard hydrophone of 1.26 volts 
and from the port hydrophone of 0.79 volt. In the 
absence of any signal the steering voltage V d is this 


4 


3 


2 



o 


or 

UJ 

Ld 

b 


i 


0 


-I 


-2 


-3 



ANGLE OF INCIDENT SIGNAL IN DEGREES 


Figure 2. Illustration of the steering voltage V d as a function of the angle 9 from which the signal comes for various 
values of signal strength. The gain is set so that the response of a hydrophone to the self noise gives a 1-v response, and 
the AVC is so designed that it does not come into play until the potential of the point P in Figure 1 reaches 2 v. There- 
after the potential of P is held down to 2 v. 


5 = 1 will be assumed in the curves to be drawn, 
since in this way the effect of the unbalance is em- 
phasized and can be more easily seen. 

The quantities R a (d) and R p (d) depend on the type 
of hydrophone used and the way it is mounted in the 
shell. For illustration, use the directional pattern 
shown in Figure 2 in Chapter 6. 

Figures 2, 3, and 4 show the voltage V d as a func- 
tion of the direction 6 from which the signal comes. 
This assumes, of course, a static situation so that the 


0.47 volt. If the signal is 3 db below the noise, the 
voltage V d is always positive so there can be no 
steering control. Under these circumstances, the rud- 
der will be always to the right and the torpedo will 
circle. Since, in all probability, the minimum value 
of V d will not at all give maximum rudder, the path 
will not be a circle of constant minimum radius, but 
will change in curvature. Nevertheless, the torpedo 
will turn around and in this way search for a target. 
This search mechanism is of use in the ExF42 mine, 


34 


TRANSFORMATION OF ACOUSTIC SIGNAL 


but in a torpedo which is expected to start out on a 
gyro-controlled course some means must be adopted 
for steering on gyro until the signal is strong enough 
to give good steering. Such a device is generally called 
a “gate” and will be discussed later. 

Other curves in Figure 2 show the voltage when the 
signal level is equal to and greater than the self noise. 
It is clear that the “stiffness” of the control, the volts 
per degree displacement from the desired course, is a 
function of the signal strength and, as will be shown 


practical manufacture, may be in one direction or the 
other and it may be made the basis of a simple search 
procedure. 

2. The unbalance of the hydrophones causes a zero 
steering voltage to correspond to angles other than 
zero. This angle becomes smaller, however, as the 
strength of the signal increases. As a consequence, 
the torpedo will tend to approach the target in a 
spiral. 

3. The unbalance in the hydrophones, or in the 



-100 -80 -60 -40 -20 0 20 40 60 80 100 

ANGLE OF INCIDENT SIGNAL IN DEGREES 

Figure 3. These curves represent the same situation as in Figure 2 except that the initial gain is 3 db lower. 


in the next chapter, it must be kept within suitable 
limits. Figure 3 shows the corresponding curves when 
the initial gain is 3 db lower. In this case the steering 
voltage produced by the self noise is only 0.23 v. 
Figure 4 shows the curves for a 3-db higher gain. 

From these curves a number of conclusions can be 
drawn. 

1. The unbalance of the hydrophones and their 
amplifiers causes the torpedo to circle in the absence 
of a signal if the gain is set high enough so that the 
differential response to the self noise gives a rudder 
deflection. This unbalance, which is unavoidable in 


self noise itself, sets a limit to the signal strength on 
which the torpedo can steer. It is clear from the fig- 
ures that the torpedo with the properties used for 
this illustration will not steer on a signal strength 
3 db below the noise, no matter how high the gain 
is set. 

4. The stiffness of the control, the volts steering 
voltage per degree off course, increases as the gain 
increases, but is not proportional to the gain. 

The above is essentially an illustration of a method 
of analysis of a typical circuit. Each circuit will have 
its peculiarities of behavior that must be understood 


DESCRIPTION OF A SIMPLE CIRCUIT 


35 


and related to the dynamical behavior of the torpedo 
to be described in the next chapter. 

In addition to the static analysis as just illustrated 
it is necessary to make a dynamic analysis of the 
operation of the control mechanism. The previous 
illustration gave the rudder position as a function of 
the direction of the sound provided the direction was 
fixed and time was allowed for the rudder to reach 
its position. If the torpedo is oscillating and the 
source of sound is moving it is important to know the 


For very slow oscillations, the rudder position for 
a given angle of sound incidence will be very close to 
that indicated by the static analysis and such curves 
as those of Figures 2, 3, and 4. For higher frequencies, 
however, the rudder will lag behind the sound and 
the amplitude of its oscillation may also diminish. 
As will be shown in the next chapter it is desirable to 
keep this lag as low as possible. 

The time lag is due to a number of factors. In the 
first place the condensers C p and C 8 take a certain 



-100 - 80 - 60 - 40 -20 0 20 40 60 80 100 

ANGLE OF INCIDENT SIGNAL IN DEGREES 

Figure 4. In these curves the initial gain is assumed to be 3 db higher than in Figure 2. 


way in which the rudder follows the direction of the 
sound. 

Perhaps the simplest way in which to make 
a dynamic analysis of the control circuit is to 
feed sound into the hydrophones artificially. This 
can be done by placing projectors close to each 
one, or by feeding a suitable electric signal into 
the hydrophone circuit. The relative intensities 
can be varied in such a way as to simulate a 
sinusoidal variation of the angle from which sound 
is incident on the torpedo. This variation can be 
carried out for a range of frequencies and the 
oscillation of the rudder observed. 


time to charge up, and until they do charge up to 
their equilibrium voltage the rudder cannot reach its 
equilibrium position. The time necessary for this 
charging can be reduced by making the condensers 
small. This, however, must not be carried so far that 
the system is too responsive to short bursts of noise. 
Another important source of time lag lies in the rud- 
der motors themselves. It takes them a certain time 
to start up and to move the rudders to the desired 
position. In general it is desired that the motors have 
a high starting torque and turn over as fast as possible 
but this possibility is necessarily limited by practical 
consideration of size and weight. 


36 


TRANSFORMATION OF ACOUSTIC SIGNAL 


8.2 OTHER TYPES OF CIRCUITS 

The above analysis applies to a circuit that makes 
use of the difference in level of the response of two 
hydrophones to determine the direction of a source 
of sound. The two hydrophones respond differently 
because of their own directivity patterns as well as 
because of the positions in which they are mounted 
on the shell. 

Another method of determining the direction of a 
sound source is by measuring the difference in time 
of arrival at two hydrophones, or, in the case of a 
sustained sound, by measuring the phase difference 
between the responses. This phase difference is 
related to the bearing 6 of the sound source by the 
equation 


where A 0 is the phase difference in degrees, d is the 
separation of the hydrophones, and X is the wave- 
length of the sound. In systems of this kind d must 
not be too much greater than X. If d = X the phase 
difference is 180 degrees for 0 = 30 degrees and the 
interpretation of the results is unambiguous only 
for | Q | < 30 degrees. Hence it is necessary to use 


either long wavelength sound or to place the hydro- 
phones very close together. 

Although this method has not been put into service 
in the United States, an extensive program of study 
on it was undertaken by the Bell Telephone Labora- 
tories 19 in the region of frequencies around 1,500 to 
2,500 c. Their work showed that the phase difference 
type of circuit could be made to operate satisfac- 
torily with a signal-to-noise ratio roughly the same 
as that needed for operation of the intensity differ- 
ence systems. However, it appeared that the self 
noise at the low frequencies studied was so high and 
so erratic that practical operation appeared doubtful. 
Presumably such a system would work satisfactorily 
at higher frequencies if suitable close locations for the 
hydrophones could be found. It does not appear, 
however, that there is any particular advantage in 
the phase-difference system. Although it might seem 
at first glance that a phase-difference system might 
operate at a lower signal-to-noise ratio than the in- 
tensity difference system, the detailed analysis shows 
this not to be the case. It will not operate effectively 
in the presence of noise whose level is significantly 
above that of the signal. In case, however, it is de- 
sired to operate at low frequencies for some other 
reason, the phase-difference system might well be 
the easier type to use. 


Chapter 9 

TORPEDO DYNAMICS AND STABILITY 


T he proper application of the acoustic signal to 
control the torpedo requires also an analysis of 
the hydrodynamic behavior of the torpedo in re- 
sponse to its rudder. This problem has been given 
considerable study, and a reasonably satisfactory 
theory of torpedo behavior is now possible. For a 
detailed treatment reference must be made to the 
volume on torpedoes, but enough of the theory will 
be outlined here to indicate the significant factors 
and the problems involved in the application of 
acoustic control. These, after all, are not greatly 
different from those involved in any automatic 
steering. 

In an elementary way, the reasons why torpedo 
and ship steering require some careful analysis may 
be roughly formulated as follows: 

1. A torpedo, or a ship, whose rudder is in the 
neutral position cannot be depended upon to travel 
steadily along a straight course, but must be steered. 
A few ships and somewhat more torpedoes, if left 
alone will move in a circle, either to the right or to 
the left. The majority run in a state that might be 
described as similar to neutral equilibrium. If by some 
means they are displaced from the course they will 
not return to it but will take up another more or less 
straight course. 

2. A given rudder position does not correspond to 
a certain direction of motion but to a certain system 
of forces and moments. In time, the forces associated 
with a rudder displacement will cause the body to 
take up a stable turning circle, but the transient 
state, before the circle is reached, is the one of im- 
portance in steering. 

3. A torpedo, or a ship, does not always travel in 
the direction in which it is headed but frequently at 
a slight angle to this direction. Hence, a clear dis- 
tinction must be made between the heading of the 
ship and the direction of motion. 

Because of such matters as the above, the detailed 
treatment of torpedo motion is a little complicated. 
Nevertheless, it is possible to understand some of the 
significant features in a qualitative way without 
treating the whole problem. In an effort to make some 
of these points clear, the problem will be approached 
in several stages. The starting point is a very crude 
idealization in which many factors are neglected, 


and a series of simplifying assumptions is made to 
permit an elementary solution of the problem. In 
successive illustrations, more and more factors will 
be taken into account until a reasonably satisfactory 
picture of the situation can be attained. The effect 
of the various factors will be pointed out at each 
stage, and the reader can pursue the more compli- 
cated analysis as far as seems to him profitable. 

9.1 CRUDEST POSSIBLE TREATMENT 
OF AUTOMATIC STEERING 

It is possible to recognize a number of the signifi- 
cant factors in automatic control by considering a 
very simple model and ignoring many of the compli- 
cations. To this end, let 0 be the angle between the 
torpedo axis and the line to the target. Then assume 
the following. 

1. The torpedo travels in the direction of its axis at 
the speed v. As pointed out above, this is not always 
true in the case of an actual torpedo, but it is close 
enough so that this assumption serves for the crudest 
possible treatment of the problem. 

2. When the rudder is at the angle 5, the torpedo is 
subject to a torque that is proportional to 8. When 8 is 
positive, the torque is in such a direction as to cause 
0 to increase. This, coupled with the previous as- 
sumption, provides the means by which the rudder 
controls the torpedo’s course. 

3. The angle 0 is small enough so that with satis- 
factory accuracy the distance h of the torpedo from 
its straight course is given by 

h = vfOdt. 

On the basis of the above simplifying assumptions, 
first the case of steering in a horizontal plane and 
second the case of depth keeping will be discussed. 

To treat the case of steering in the horizontal plane, 
assume that the reference direction is the direction 
from the torpedo to the source of sound. In the case 
of a torpedo under gyroscope control, the reference 
direction is merely the direction of the course pre- 
scribed by the gyroscope. Then the following is 
assumed. 

4. The control mechanism is such that the rudder 
deflection is proportional to the angle by which the 




37 


38 


TORPEDO DYNAMICS AND STABILITY 


torpedo departs from the prescribed course and is in 
the direction to restore the torpedo to its course. 

8 = —ad. 

Upon the basis of this assumption, combined with 
the first three and the fact that a body changing di- 
rection in its underwater motion experiences an op- 
posing torque proportional to its angular velocity, 
the equation of motion is 

Qd = — Kd-\- ad, 
or 

Qd + Kd + aad = 0. (1) 

In this equation Q is the effective moment of inertia 
of the torpedo and the entrained water. Treatments 
of the flow of a perfect fluid around an ellipsoid sug- 
gest that this is roughly twice the moment of inertia 
of the torpedo itself. K depends strongly on the shape 
of the torpedo. For a sphere it would be negligible, 
but for a torpedo it can be estimated from model 
studies and the observed torpedo behavior. Esti- 
mates are available for some of the standard tor- 
pedoes. The torque is a in pound feet experienced by 
the body when the rudder angle is one radian. Its 
magnitude clearly depends on the rudder area, the 
shape of the afterbody, the fins, the rudder location, 
and the velocity v. The constant a represents the 
stiffness of control. It represents the ratio of the 
rudder angle to the angle of departure of the torpedo 
from the prescribed course d. 

Since the constant K is always positive, equation 
(1) always represents a stable damped motion. No 
matter what may momentarily disturb the tor- 
pedo, it will eventually settle down to the prescribed 
course, 0 = 0. Hence K may be called a stabilizing 
factor. Other things being equal, the larger the value 
of K the less the torpedo will oscillate about its pre- 
scribed path. 

On the other hand, if K is large and aa is small, 
the torpedo will only recover from a disturbance ex- 
ceedingly slowly. Furthermore, if the target is mov- 
ing, a certain minimum value of aa is necessary to 
insure that the torpedo follows the change in path 
with sufficient accuracy to finally strike it. Hence to 
insure the type of tracking that is desired, a suitable 
balance must be struck between the “stiffness of 
control” aa and the damping K. 

To get a similar very crude and qualitative treat- 
ment of torpedo depth keeping it is only necessary to 
include the effect of the departure h from the desired 
depth on the rudder position. Let d be the angle by 


which the torpedo points above the horizontal, h the 
distance by which it is above the desired depth, and 
assume the following. 

5. The control mechanism is such that 

8 = —ad — (3h. 

The equation of motion is then 

Qd = —Kd — aad — a/3h, 
or 

Q d -\- Kd + aad T - afivd = 0, (2) 

by making use of assumption 3. This is now a third 
order differential equation, and the condition for 
stability is slightly more complicated than before. 
Here again the stability referred to is the property 
that any solution of equation (2) approaches 0 = 0 
as time goes on. 

The first condition for stability is that all of the 
coefficients be positive. This is automatically satis- 
fied in the case at hand since a and /3 are made posi- 
tive in the construction of the depth-keeping mechan- 
ism. The other condition for stability is 

°Y > Q. ( 3 ) 

/3v 

Here, as before, K is a stabilizing factor, but a is 
also. An increase of the rudder response to the angle 
of tilt increases the stability of the system. On the 
other hand, an increase in /3, or in v , may make the 
control unstable. In particular, if an attempt is made 
to control the depth of the torpedo by using the 
water pressure only without reference to the tilt, i.e., 
by making a = 0, the torpedo will be unstable, and 
will not keep its depth satisfactorily. 

In the normal torpedo depth control there is a 
pendulum so arranged that a tilt of the torpedo af- 
fects the rudder position somewhat according to 
assumption 5 and this is a very essential part of the 
depth-control mechanism. It was the introduction 
of this pendulum that made satisfactory depth keep- 
ing possible. From the above simplified assumptions 
it might be expected that a mechanism responsive to 
h would serve as well as a pendulum. It must be re- 
membered, however, that the above assumptions 
are extremely crude, and it will be shown later why 
such methods are subject to important limitations. 

As in the case of steering in the horizontal plane, 
there are two opposing sets of factors. There are first 
the stabilizing factors, the damping coefficient K 
and the response to tilt aa. These tend to keep the 
torpedo on a steady course. Then there is the re- 


INCLUSION OF TIME LAGS IN THE CONTROL SYSTEM 


39 


sponse to departures from the correct depth ( a(3v ) 
which reduces the stability but is necessary to correct 
disturbances in depth and to keep the torpedo at the 
correct depth. These two properties must be cor- 
rectly balanced for satisfactory depth keeping. 

In both of the cases just discussed the factors 
leading to stability and the factors leading to precise 
steering or depth keeping, are opposing, and a suit- 
able compromise between them must be made in the 
design. 


6. The control mechanism is such that 
5(2) = - <xd(t - r). 

The use of this assumption gives the differential 
equation 

Qd Kd = a8. (4) 

Since 5 is no longer assumed to be simply propor- 
tional to 0, this is of a different type from equation (2) 
and one whose solution must be approached in a dif- 



Figure 1. Phase in degrees as a function of a> for several values of the time lag r. The heavy curve gives log - 
These curves assume Q = 1,500 slug ft 2 , K = 10 3 lb-ft per radian per sec, and a = 2 X 10 3 lb-ft per radian. I 


9.2 INCLUSION OF TIME LAGS IN 
THE CONTROL SYSTEM 

In the above treatment of steering and depth con- 
trol the simplifying assumption was made that the 
rudder position followed the acoustic or other signal 
exactly, without any time lag. This is rarely practi- 
cable or even possible. In the case of the circuit 
scheme outlined in Chapter 8, the condensers take a 
certain time to charge up so that the voltage V d lags 
a certain amount behind the acoustic signal. In addi- 
tion, the inertia of the rudder motor and the rudder 
itself, and the time constant of the circuit involving 
them, cause an additional lag of the rudder behind 
the voltage V d . Hence, the crude picture is made one 
step closer to the actual situation by assuming for 
steering in the horizontal plane, that 


ferent way. One method of investigating the proper- 
ties of the solution is as follows. 

Assume that a periodic solution exists and then 
find the conditions imposed upon it by the differential 
equation (4). Hence let 

5(2) = 5 0 e iw< and 0(2) = 0 0 e iut . 

Since only the relative phases are important, assume 
that 5 0 is real and 0 O is complex. Substitution in equa- 
tion (4) leads to 

; = --Hr~nr (5) 

To satisfy the relationship assumed in assump- 
tion 6 it is necessary that 

00 , _ 1 
5 0 a 




40 


TORPEDO DYNAMICS AND STABILITY 


and that 


These conditions can be represented by curves of the 
type shown in Figure 1. For illustration, it is assumed 
that 

Q = 1,500 slug feet 2 , 

K = 10 3 pound feet per radian per second, 
a = 2 X 10 3 pound feet per radian. 


as co — > co , but as co — > <» , d 0 /d 0 — > 0. At very high 
frequencies the amplitude of 6, because of a unit 
amplitude of 5, is so small that it will not maintain 
the motion of 8, and the whole oscillation will die out. 

On the other hand, for r = 0.1 the phase is zero 
at co = 2.55 "where the logarithm of the absolute 
value is —0.7. Hence, if the stiffness is high enough, 
i.e., a = 5.0, the motion of the body, through the 
control system, will maintain sufficient rudder mo- 
tion to, in turn, maintain the body oscillation. If the 



CO IN RADIANS / SEC 

Figure 2. These curves are similar to those in Figure 1 but with K = 10 4 lb-ft per radian per sec. 


The value assumed for Q, the moment of inertia, is 
of the order of magnitude of that for the Mark 13-2 
torpedo, as is a, the rudder torque constant. To pro- 
vide an illustrative case, K is taken about 1/10 of 
the probable value for the Mark 13-2. The heavy 
curve in Figure 1 shows the. absolute value of d 0 /8 0 
as a function of w for values of co between 0.1 and 10. 
The ordinate as indicated in the left-hand scale is 
log | Oo/8u | . The curve shows that if a = 1, equa- 
tion (6) can be satisfied only for co = 1.06. For a 
stiffer control, a = 2, log 1/a is —0.3, and co = 1.57 
is the only possible value. 

But it is also necessary that the left-hand side of 
equation (6) be real and this requires a suitable 
value of r. The phase angle is also shown in Figure 1 
for various values of r. For r = 0 the phase is al- 
ways positive, which indicates that there can be no 
periodic solution. This corresponds to the stable 
cases treated previously in which each solution con- 
tains a decreasing exponential term. It could be de- 
scribed by saying that the phase approaches zero 


stiffness is greater than 5.0 or the time lag is greater 
than 0. 1 sec, the motion will build up larger and larger 
oscillations. 

Here, then, is another destabilizing factor. The 
time lag in the control system as well as the stiffness 
of the control both tend to lead to oscillations that 
must be opposed by the hydrodynamic damping. 
Since the time lag in the control mechanism cannot 
be reduced to zero in most cases, a limit is set on the 
stiffness of control that can be used. 

Figure 2 shows the corresponding curves for a 
damping factor 10 times as great. With this larger 
damping factor greater stiffnesses as well as greater 
time lags can be permitted and still maintain stable 
steering. 

9.3 FAIRLY COMPLETE TREATMENT 
OF AUTOMATIC STEERING 

To get anything like a quantitative treatment of 
torpedo steering it is necessary, as was pointed out at 




FAIRLY COMPLETE TREATMENT OF AUTOMATIC STEERING 


41 


the beginning of the chapter, to distinguish clearly 
between the direction in which the torpedo is point- 
ing and the direction in which it is moving. To do 
this use the notation indicated in Figure 3. The path 



Figure 3. Illustration showing the notation used to 
describe the motion of a torpedo in the horizontal 
plane. 

of the center of mass is taken as the torpedo tra- 
jectory and all forces and moments are considered 
as referred to this center. The angle 0 then describes 
the direction of the course, or, trajectory, with refer- 
ence to a fixed direction while the angle \f/ is the 
angle between the axis of the torpedo and the course. 
The rudder angle 8, as before, is positive in the direc- 
tion in which it tends to increase 0. 

As is shown in books on hydrodynamics, the iner- 
tia of a body immersed in water includes that of a 
certain amount of entrained water. In the case of a 
body somewhat ellipsoidal in shape as is a torpedo, 
the momentum can be described by the use of two 
masses. One mass applies to motion along the torpedo 
axis and the other to motion transverse to the axis. 
Referring again to Figure 3, i is a unit vector in the 
direction of the axis and j a similar unit vector at 
right angles to it . Then the vector momentum of the 
system is 

M = Miv cos \p i — M 2 v sin \J/ j. (7) 

Since M 2 is usually larger than Mi the momentum 
is not in general parallel to the velocity. In comput- 
ing the force necessary to change this momentum, 
account must be taken of the change in direction of 
the unit vectors, i and j, as well as the change of the 
scalar terms. This leads to the two forces, longi- 
tudinal and transverse, 

F t = Mi { v cos \f/ — v sin \J/ij/} + M 2 v sin \l/(\J/ + 0) 

F t = M\v( + rj>) cos ^ — M 2 { v sin + v cos W } . 

If now it is assumed that v can be neglected, and that 


both angles and their rates of change are small quan- 
tities, Fi contains only terms of the second order of 
small quantities so may be considered as zero for 
the case of operation at constant speed. To the same 
approximation the transverse force becomes 

F t = Miv(d -f- \p) — M 2 v\f/. (8) 

To write the equations of motion it is necessary to 
define a number of hydrodynamic coefficients. 20 The 
effective forces follow. 

1. The propeller thrust T. This may be treated 
effectively as a constant along the axis of the torpedo. 

2. The drag force D. This can be expressed in 
terms of a drag coefficient Cd, the cross section of the 
torpedo in square feet A, the density of the water in 
slugs per cubic foot p, and the velocity in feet per 
second v. 

D = C I> A-v 1 ’-- (9) 

The drag force is opposite to the velocity v and so 
has components both parallel and perpendicular to 
the torpedo axis. 

3. The cross force L. This is perpendicular to the 
velocity v, and can be expressed in terms of a coeffi- 
cient C L. 

L = C l A^- (10) 

4. A moment M h described by a moment coeffi- 
cient Cm . 

M„ = C M Al P -v> (11) 

where l is the length of the torpedo. 

5. A damping moment Md proportional to the 
angular velocity of the body and described by a co- 
efficient Cr 

M d = -C K PA p ^v(e ++)■ (12) 

6. A transverse force associated with the damping 
moment and described by C F . 

F d = C F lA?v(6 + *)• (13) 

The drag, cross force, and the moment M h can be 
measured in a water tunnel or a wind tunnel, so that 
the corresponding coefficients can be determined, at 
least approximately. They are found to depend on 
the angle of yaw \f/ as well as on the rudder angle 8. 
Such determinations are made, of course, with the 
center of mass moving in a straight line relative to 


42 


TOPEDO DYNAMICS AND STABILITY 


the water and it is assumed that the same values are 
approximately correct when the motion is in some 
other path if the radius of curvature is large enough. 
The coefficients C K and C F are taken as constants, 
and the presence of these terms may to some extent 
compensate for the errors in the assumption that 
equations (9), (10), and (11) apply when the body is 
moving in a curved path. 

Taking components along the torpedo axis 

Fi = T — C D A% 2 cos \f/ + C lA-v 2 sin \p + 

Z Z 


/ 2Af L _ CA 
\p4p v ) 


— )(« + *)- 


- Cot - C L = 0- (15) 


20 Cl 2 - 

— ~0 + P) H — —(6 + ~ CMl — 0- 

pAv 2 v 


(16) 


The coefficients C K and Cd can be regarded as con- 
stants. C l and C M , on the other hand, depend on the 
angle \p and the rudder angle 8. Observations show 
that a linear dependence is an adequate approxima- 
tion for small angles, so let 


Cl — ci\J/ — b8 



(17) 


-20 


-40 <f> 


-IOO 


0.2 0.3 0.4 0.6 0.8 2 

COIN RADIANS /SEC 


6 8 10 


Figure 4. The value of log |^o/5o| as a function of w. The heavy line gives the logarithm of the absolute value and the 
lighter line gives the phase in degrees. 


Since the angles are assumed small, the terms con- 
taining sin \p can be neglected compared with the 
first two. Equating the first two to zero gives the 
speed in terms of the propeller thrust when the vari- 
ations in v are neglected. This equation can then be 
omitted from the further considerations of steering. 

The components of force perpendicular to the 
torpedo axis lead to 

F t = C d A% 2 sin \J/ + C l A-v 2 cos \ f / + 

z z 

C F IA -v{6 cos \J/’ (14) 

Z 

All of these terms are of the same order because C l 
is proportional to \p. Using expression (8) for F t in 
equation (14), and equating the moments to the 
moment of inertia times the angular acceleration 
leads to two differential equations of motion 


where a, b , c, and / are constants. When these forms 
are inserted in equations (15) and (16) the result can 
be written 

A id -f- B\j/ B 2 \p = — 8 (18) 

A 2 0 H~ A^d + Ayjs + Az^p H - Bzp = 8, 

where 

/2Mi _ Crl\ 

\ P Av v ) 


A 2 


2 Q 

P Av 2 fl 


Az 


Ck 

vf 


/ 2Mi _CfI _ 2 MA 

\pAv v pAv/ 


FAIRLY COMPLETE TREATMENT OF AUTOMATIC STEERING 


43 


B , = 


B* = 


( Cd ~h cl) 


stants will be assumed. They are approximately those 
applicable to the Mark 13 torpedo with a shroud 
ring. 


The above equations give the motion of the tor- 
pedo in response to a prescribed rudder angle 8, and 
it will be assumed that the same equations hold 
when 8 is changing. To apply acoustic control, 8 must 
be made some function of (0 + \p). A fair representa- 
tion can be obtained by making 8 proportional to 
— (0 + id with a time lag as was done in a simpler 
case above. 

To proceed with the study of equation (18) it is 
convenient to assume a periodic value of 8, and to 
find the corresponding periodic solution for 0 and \p. 


Mi = 67 slugs 
M 2 = 116 slugs 
v = 50 ft per sec 
A = 2.75 sq ft 
= 2 slugs per cu ft 
= 0.50 
= 1.10 

= 2.23 per radian 
= 0.114 per radian 


P 

Ck 

Cp 

a 

b 


c = 0.281 


/ = 0.0573 
l = 13.5 ft 
C D = 0.10 
Q = 1,500 slug ft 2 
Ai = 1.67 
A 2 = 0.282 
A 3 = 2.36 
B i = -5.73 
B 2 = -20.4 
B 3 = -4.90 


Figure 4 shows the value of \f/ 0 /8 0 as a function of 
If the rudder is turned back and forth with a 



-90 


-no 


-130 


0.1 


0.2 


0.3 0.4 


0.6 Oj8 I 2 

CO IN RADIANS/ SEC 


Figure 5. The value of log |0 O /5 O | as a function of a>. As previously the heavy line shows the absolute value and the 
lighter line gives the phase. 


Hence let 8 = 6 = d 0 e lO3t , \f/ = \f/ 0 e tut where 5 0 

is real but 0 O and \J/ 0 are complex. Insertion in equa- 
tion (18) then leads to 

_ {Ai A 3 i(j)A2\ 

e 1= { A3 + Bl + t ( MA2 - gl T g3 )| 

So A 

A = [A \B 3 — B 2 A 3 — co 2 (^ 4 i — .B 1)^12} 

+ iu { (Ai — B 1)^.3 — A 2 B 2 } 

For illustration the following values of the con- 


sinusoidal motion having an amplitude of 10 degrees 
and a frequency of 0.0159 c (co = 0.1) the angle i 
will vary sinusoidally with the same frequency, a 
phase lag of 3 degrees, and an amplitude of about 
1.0 degree. At this slow rate the amplitude of ^ is 
nearly the equilibrium value of \J/ if the torpedo were 
in a steady turn with the given rudder amplitude as 
a fixed rudder angle. 

If the frequency of the rudder motion is made ten 
times as great (/ = 0.159, a> = 1.0) the amplitude 
through which \p oscillates is still about the same but 
the phase lag has become some 28 degrees. When the 
frequency is again multiplied by 10 (/ = 1.59, w = 10), 


44 


TORPEDO DYNAMICS AND STABILITY 


the amplitude of \l/ is 0.18 degree and the phase 
lag has become over 90 degrees. At this frequency 
the torpedo turning is always in the transient state 
and steady turning conditions never get a chance to 
develop. 

The behavior of the angle 0 , is a little different as 
is shown in Figure 5. For low frequencies it can be- 
come much larger than the rudder angle since the 
torpedo keeps on turning as time goes on. At the 
frequency of 0.0159 c (co = 0.1) the amplitude of 0 


oscillates with a large amplitude. On the other hand 
if the rudder is moved rapidly back and forth \Jy fol- 
lows it to some extent, but 0 is hardly affected at all. 
When a torpedo starts to turn, \j/ increases first and 
then 0 follows, but if the direction of the motion is 
soon reversed there is not time for any significant 
variation in 0. 

An acoustic control will respond only to the sum of 
the angles (0 + \J/), hence it is important to see how 
this behaves. Figure 6 shows the amplitude and phase 



GJ IN RADIANS /SEC 


_i 

I 

Ui 


Figure 6. The value of log |(0 O + i/'o)/5o| as a function of o>. In addition the phase of a quantity that lags behind the 
sum of the angles by a constant time lag r. This time lag corresponds to a greater phase lag as the frequency increases. 


is about 6.3 times that of the rudder angle. If the 
rudder is being oscillated with an amplitude of 
10 degrees, the 63-degree amplitude of 0 would con- 
tradict the original assumption that all angles are 
small and the differential equations would no longer 
apply. If, however, the rudder amplitude were made 
small, say 2 degrees, the method would be applicable. 
Under these circumstances the angle 0 lags some 
92 degrees behind the rudder angle <5. As the fre- 
quency is increased the amplitude of 0 becomes 
rapidly smaller so that for co = 10 it is only 1/50 
that of 8 and somewhat more than that of \f/. The 
phase lag is also near 180 degrees. 

These curves illustrate the ways in which the two 
angles 6 and \p behave when the rudder is oscillated. 
If it is swung slowly, \f/ reaches a limiting value but 9 


of the negative of the sum, — (0 O + ^o) /$o- The phase 
is given by the curve marked r = 0. If the position 
of the rudder is made proportional to — (0 O + ^o) 
and there is no time lag, the system will be stable and 
a large stiffness can be used. This is shown by the 
fact that the curve marked r = 0 crosses the axis of 
zero phase shift at co = 2.2 where log | (0 O + \f/o)/8 0 | 
= —0.6. If, however, the almost unavoidable time 
lag is taken into account, restrictions are at once 
placed on the stiffness. The curves marked r = 0.1 
and r = 0.4 show the phase of the rudder oscillation 
if it lags at time r behind (0 + \J/). If the time lag is 
0.4 sec the phase becomes zero for co = 1.3. This 
means that if the rudder is oscillated at such a rate, 
the motion of the body will be such as to just main- 
tain this oscillation when the coefficient of propor- 


GENERAL CONCLUSIONS 


45 


tionality is properly chosen. Figure 6 shows that for 
co = 1.3 the amplitude of the torpedo motion will be 
about 0.4 as much as that of the rudder. If then the 
stiffness is such that 0.4 degree torpedo angle pro- 
duces 1 degree rudder angle the motion will just be 
maintained. A smaller stiffness will lead to the type 
of stability in which the oscillations will die out 
while a greater stiffness would cause the oscillations 
to increase without limit. 

9.4 GENERAL CONCLUSIONS 

The above examples illustrate the way in which 
the response of a torpedo to a control system can be 
analyzed. The constants describing the behavior of 
the body must be obtained from measurements in a 
water tunnel or a wind tunnel and by a study of the 
turning characteristics of the body. The description 
of the control system including such things as the 
stiffness and the time lag must be determined either 
from actual measurements on the system or by calcu- 
lations on the basis of the design. The above illus- 
trations are cases of a “proportional” control, in 
which the rudder position is proportional to the dis- 
placement of the torpedo from the prescribed direc- 
tion. It is possible, however, to apply similar analysis 
to cases in which the rudder is put hard over in either 
one direction or the other. The details of such sys- 


tems will not be worked out here but in general it 
appears that the limitations on stiffness and time lag 
are a little more severe than with the proportional 
systems. 

In general it is clear that “steering” and “sta- 
bility” are somewhat contrary requirements. To 
steer easily a torpedo should respond quickly and 
vigorously to any departure from its prescribed 
course. This is produced by a stiff control system, by 
large rudders, and by a short radius of turn. On the 
other hand these factors tend to make a body over- 
shoot its course and to oscillate widely about the de- 
sired path. To provide stability on a course, the cor- 
rection of departures from it must be tempered by 
limiting the stiffness and the rudder area, by limiting 
the sharpness of turn, and in addition, by shaping the 
body so as to produce large damping forces. A suit- 
able compromise must be reached between the factors 
leading to good steering and those leading to stability 
on the course. 

In addition to the opposing sets of factors leading 
to good steering and to stability, a time lag in the 
control system always leads toward instability with- 
out producing any improvement in steering. As a 
consequence, it is necessary to keep the time lag as 
small as possible since stability in the presence of a 
large time lag can be obtained only at the expense of 
steerability. 




Chapter 10 

MISCELLANEOUS PROBLEMS 


I n the detailed application of the principles that 
have been described in the previous chapters 
there are many points that must be given careful 
study, and engineering skill of the highest order must 
be put into the design in order to produce a practi- 
cal, workable, and rugged mechanism. Solutions of 
many of these problems can be recognized in a study 
of the accepted designs and specifications, and will 
not be discussed here. In this chapter only a few of 
the problems that are of general importance will be 
mentioned. 

10.1 BALANCING THE AMPLIFIERS 

In the circuit described in Chapter 8 it is neces- 
sary that the two amplifiers be closely balanced and 
that this balance be maintained over a very wide 
range of signal strengths. The most obvious way to 
try to do this would be merely to design and con- 
struct both amplifiers so carefully that the balance 
could be established once and for all, and would not 
change. This, however, is difficult for the large values 
of gain required, the type of service under which a 
torpedo must stand up, and the necessity of occa- 
sional changes of tubes. It has been used in the 
ExFER42 mine as developed by the General Electric 
Company and manufactured by the Leeds and 
Northrup Company. In this circuit the gain of the 
amplifiers that must be accurately balanced is con- 
siderably less than in other circuits and, although 
extensive service tests of this device have not yet 
been made, it appears to be capable of satisfactory 
operation. 

Two other methods have been successfully used in 
acoustically controlled torpedoes and will be briefly 
indicated here. 

10.1.1 A Switching Scheme 

This method consists in using the same amplifier 
for both hydrophones and switching the amplifier 
rapidly from one to the other. In the first experi- 
mental models of the ExF42 mine this switching 
was done mechanically by a rotating commutator. 


Figure 1 shows the schematic arrangement of such 
a system. The two commutators are driven by the 
same motor so that both the amplifier and the recti- 
fier are switched from one hydrophone circuit to the 
other. The rate at which the commutation takes 
place must be adjusted to the other properties of the 
control circuit. In the experimental model of the 
ExF42 mine the commutation was carried out at a 
rate of 40 c. It was then necessary to make the time 
constant of the A VC so long that it did not act on 
the difference in response of the two hydrophones 
and so tend to smooth out this differential. This 
commutation frequency is also a frequency of modu- 
lation of the incoming signal so that it sets a limit 



Figure 1 . Schematic diagram of a mechanical switch- 
ing system by means of which a single amplifier and rec- 
tifier can be used in two channels. 

to the sharpness of tuning possible in the amplifier 
circuit. With suitable adjustment of the various 
time constants this commutation system worked 
quite satisfactorily although it is not very suitable 
for Service use. 

In the final form of the ExF42 mine the commu- 
tation was carried out electronically and at a con- 
siderably higher rate. This system, free from moving 
parts, was felt to be much more satisfactory for field 
use. In general, the switching method solves the 
problem of balancing the amplifiers, but it intro- 
duces additional difficulties associated with the 
switching equipment and the proper adjustment of 
time constants. However, these problems can be 
satisfactorily solved as is evidenced by the per- 
formance of the ExF42 circuit. 


46 


GATE OPERATION 


47 


10.1.2 A Pilot Channel System 

This method, which has shown satisfactory per- 
formance in experimental operation, provides moni- 
toring of each channel by a signal of a frequency that 
can be filtered out of the amplifier output and used 
to operate volume controls. Figure 2 shows a sche- 
matic diagram of this kind of system. A single 2-kc 
oscillator feeds a signal into both circuits along with 
the signals from the hydrophones. After passing 
through the amplifiers, the different frequency sig- 
nals are separated and the 2-kc signals are main- 


signal reaches a predetermined level at which ver- 
tical steering begins. Operations of this kind can be 
carried out by means of relays that are then called 
“gates.” It is possible to operate a gate on the sound 
differential as well as the sound level so that the 
torpedo steers on its gyro course until the differen- 
tial between the hydrophones attains a predeter- 
mined value. This type of gate permits operation at 
a somewhat lower level and also keeps the torpedo 
under gyro control as long as it is headed closely at 
the target. 



Figure 2. Block functional diagram of a pilot channel 
Sound Laboratory. 

tained constant and equal by their respective AVC 
systems. The overall gain of the amplifiers is con- 
trolled by changing the level of the 2-kc input into 
both channels. This circuit also requires careful 
design but it has shown satisfactory operation in 
experimental models built by the Harvard Under- 
water Sound Laboratory [HUSL]. 

10.2 GATE OPERATION 

In many cases it is desirable to have the torpedo 
operate on a preset gyro course until the sound from 
the target has become sufficiently intense to give 
adequate control. In other cases it is desirable to 
steer in azimuth only, at a fixed depth, until the 


amplifier circuit as developed by the Harvard Underwater 

10.2.1 Level-Operated Gate 

The average sound level is given by the potential 
at the point P in Figure 1 of Chapter 8. This can be 
connected to a relay through a triode if desirable, in 
such a way that the relay operates when V p reaches 
a predetermined value. The relay can be such that 
it will open again when V p drops, or it can be such 
as to lock closed after it has once operated. When a 
level-operated gate is used to transfer from gyro 
steering to acoustic steering, the adjustment must 
be such that the gate does not operate on self noise, 
for then the torpedo would go into a circle as de- 
scribed in Chapter 8 and would not continue to 
approach the target. On the other hand, the gate 



48 


MISCELLANEOUS PROBLEMS 


should not require such a high level to operate it that 
the effective homing range is too short. 

If the torpedo self noise were entirely constant in 
time, so that the potential V p due to it were main- 
tained constant, the gate could be set to operate at 
a level only a trifle above that of the self noise. 
However, this is rarely the case. The noise tends to 
fluctuate and may contain modulation frequencies 
corresponding to the revolutions of the propeller. 
It is then necessary to set the gate to operate suf- 
ficiently high above the average background level 
that it is not operated, or at least is not often oper- 
ated, by peaks in the noise level. To set the gate 
above the highest peak that can occur would prob- 
ably mean setting it so high that the acoustic range 
would be too small, but the gate circuit can be de- 
signed so that its time constant will not permit it to 
operate on sharp peaks at all but only when the 
level is maintained for a sufficient time. If the time 
constant were indefinitely long, the circuit would 
respond only to the long time average level, but it 
can be made reasonably short and still respond to 
fairly representative average levels. It appears prac- 
tical to set a gate to operate at some 5 or 6 db above 
the average self-noise level and get satisfactory re- 
sults. If the self-noise level is low enough, this 
wasted 5 or 6 db is of little importance, but since 6 db 
corresponds to a factor of 2 in the range, there are 
cases where this may make the difference between 
satisfactory homing and a range that is too short to 
be of much use. 

In designing a circuit with gate operation, careful 
attention must be paid to the interrelationships of 
the various time constants. These are associated 
with the steering circuit, the A VC circuit, and the 
gate circuit. In the simple circuit shown in Figure 1 
of Chapter 8 the time constant associated with V d is 
the same as that of the A VC, but this need not be 
the case. Presumably, the maximum gain of the 
amplifiers will be set so that the A VC will not be 
called into operation by the torpedo self noise and 
the time constant of the steering circuit will then be 
made long enough to average out fluctuations in 
this noise but not long enough to introduce signif- 
icant instability into the steering. An additional 
length of time constant can then be introduced into 
the A VC since the average response of the two hydro- 
phones will change more slowly than the differential 
when the body is oscillating about its course. 

In some cases it has been found that the noise 
from a surface ship is strongly modulated at a rather 


low frequency corresponding to a propeller blade 
frequency. In such a case it may be undesirable to 
use a time constant long enough to average out this 
modulation because of the sluggishness that would 
be introduced into the system. On the other hand, it 
is also impossible to have the gate relay opening and 
closing several times a second until the minima are 
sufficient to operate it. Under such circumstances, 
the gate may require still a third time constant to 
control its operation. 

10.2.2 Differential-Operated Gate 

It is also possible to arrange a gate to operate on 
the differential response of the hydrophones as rep- 
resented by the voltage V d . If the response to self 
noise were entirely symmetrical, the self noise would 
never cause any differential voltage. Since, however, 
the hydrophone circuits may be a trifle out of bal- 
ance, and since the noise itself may be stronger at 
one hydrophone than at the other and may also 
pulsate differently on one side than on the other, 
allowance must be made for a differential due to the 
self noise. The minimum that must be allowed for 
must be determined by experiments on each par- 
ticular kind of torpedo, but some preliminary tests 
have suggested that 3 to 4 db might be ample. In this 
case the torpedo could start to home on a signal only 
1 or 2 db above the background if it were coming 
essentially from one side. This method has not yet 
had extensive service tests, but considerable ex- 
perimental work is being done on it. 

10.3 RUDDER OPERATION 

The rudders of a torpedo may be operated in a 
number of ways. Most conventional, nonhoming 
torpedoes use compressed air for this function, but 
since acoustic homing devices require electric circuits 
and electric energy, electrical rudder operation is 
frequently used. One method makes use of rudder 
motors. This is suggested by analogy with the steer- 
ing of large ships where motors are often used to 
move the rudders. The first ExF42 mine used this 
system, and the experience gained in this way has 
brought to light a number of disadvantages. 

In order to get sufficient power without a very 
heavy motor it is necessary to operate the motor at 
high speed, and to drive the rudders through a re- 
duction gear. This tends to produce noise and may 
be the limiting source of noise in the ExF42 mine. 
Furthermore, it is difficult to get sufficient quickness 


STEERING IN DEPTH 


49 


of response because of the time necessary to acceler- 
ate the motor and gear system. This latter factor 
was found to be one of the principal elements in the 
time lag of the control system. On the other hand, 
rudder motors seem well adapted to a positioning 
system, since they can move the rudders to the de- 
sired position and then stop. 

Another method that has been used in the Ex20F 
torpedo involves the use of solenoids for pulling the 
rudders one way or the other. This provides quick 
action but involves the use of heavy currents, which 
are somewhat troublesome to handle, and greater 
weight. For a system in which the rudders are thrown 
one way or the other, this may be the simplest pro- 
cedure but the use of solenoids for a positioning 
system seems less straightforward, although it can 
be done. 

10.4 STEERING IN DEPTH 

For an antisubmarine torpedo the problem of 
steering in depth as well as in azimuth must be 
solved. Furthermore, the discovery of cavitation as 
a principal source of torpedo noise suggested at once 
that the acoustic homing range could be much in- 
creased by normally using a running depth of from 
50 to 100 ft. To attack a surface target it is nec- 
essary for the torpedo to come up at the target in 
just the right way to hit it. This presents problems of 
dynamics and control quite different from those met 
with in ordinary torpedo practice, and problems that 
are not yet fully understood. Just a few aspects of 
the matter will be mentioned here. 

One of the difficulties arises in connection with the 
stability of the torpedo with reference to heel, par- 
ticularly in the case of a torpedo with a single pro- 
peller. In this case the propeller torque is balanced 
by the torque associated with a displaced center of 
mass, and the torque is balanced only when the 
torpedo axis is horizontal. If the torpedo should 
travel vertically upward, there would be no torque 
compensation at all, and the torpedo would begin to 


spin. This suggests that means must be provided to 
limit the angle of climb or dive to such small value 
that no untoward spinning can occur. 

Another associated difficulty can be illustrated by 
considering the case of motion in a circle. If the 
rudders are put hard over to port, the torpedo takes 
up a circular motion and continues to turn in this 
circle as long as power is available. If, however, the 
elevators are put hard up, the torpedo starts to turn 
in a circle in a vertical plane. Before it has completed 
180 degrees it is upside down and will probably turn 
over, whereupon instead of completing its circle it 
starts up again. 

Another difficulty with vertical steering is the 
presence of the surface. When maneuvering in a 
horizontal plane, the torpedo can oscillate from side 
to side of its course and if it misses the target it can 
turn around and try again. If, however, it oscillates 
too widely in the vertical plane, it may jump clear 
out of the water and will return by falling back 
rather than by following the course prescribed by 
the controls. 

It seems that most of the methods thus far used 
for overcoming or avoiding these difficulties in verti- 
cal steering can be described as means for reducing 
maneuverability in the vertical plane and requiring 
the sharp turning to be done by means of the rudders 
for steering in azimuth. This can be done by means 
of a climb angle limiter in the form of a pendulum 
that takes control when the inclination exceeds a cer- 
tain maximum and cuts out or reduces the effect of 
the acoustic signal. Another method consists in re- 
quiring the vertical acoustic control to oppose the 
hydrostatic and pendulum depth control so that the 
torpedo can climb steeply only under a very strong 
signal. To keep the torpedo from rising too soon the 
sensitivity in the vertical channel may be reduced 
below that in the horizontal channel. Most of these 
methods are palliative, and it seems that the problem 
of steering in depth still awaits a complete analysis 
and solution. 


Chapter 11 

SIGNAL AND NOISE LEVELS IN HOMING BY ECHO RANGING 


I n a torpedo to be controlled by echo ranging, a 
sound signal is radiated by a projector located in 
the torpedo, and the echo returned from a reflecting 
body in the neighborhood is used to steer the tor- 
pedo. Some of the problems involved in this type of 
control may be listed as follows. 

1. The projector must radiate into the water 
enough sound energy so that the returned signal can 
be distinguished from the background and self noise. 
The signal must also be radiated in a wide enough 
solid angle to reach any expected targets. 

2. The listening mechanism must be such as to 
identify the direction from which the echo is re- 
turned. It must not be put out of commission by the 
periodic operation of the powerful transmitter, and 
it must, as far as possible, distinguish the desired 
echo from undesired echoes such as reverberation 
and bottom echoes. 

3. The steering mechanism must be such as to 
direct the torpedo on the basis of the intermittent 
information obtained from the echoes. This is a 
somewhat different matter from steering on the con- 
tinuous information available to a listening tor- 
pedo. 

The remainder of this chapter will be devoted to 
a discussion of the problems of the projector. 

Let P represent the total power in watts radiated 
by the projector into the water. One of the principal 
advantages of the echo-ranging method is the fact 
that this quantity is to some extent under the con- 
trol of the designer. In general it is desired to make 
this as large as possible, so that some attention must 
be given to the factors that limit it. One of these 
factors is cavitation at the surface of the projector. 
If the minimum pressure in the sound wave gets 
down to the neighborhood of the vapor pressure, 
cavities will be formed into which the liquid evapo- 
rates and gas comes out of solution. This begins to 
occur in the neighborhood of watt per sq cm in 
water at about two atmospheres pressure. Observa- 
tions by the Bell Telephone Laboratories [BTL], 
however, have indicated that this figure can be much 
exceeded for very short pulses. Apparently, if a pulse 


length of about 1 to 5 msec is used, the cavitation 
does not have time to develop and more power can 
be radiated than with longer pulses. The BTL projec- 
tor for trial in the Mark 14 torpedo was thought to 
be radiating close to 1,000 watts. This is much above 
the x watt per sq cm and is possibly near the prac- 
tical limit for tranducers suitable for use in a tor- 
pedo. 

As indicated in the previous paragraph, more en- 
ergy can be radiated from a large projector than 
from a small one. A large projector, however, tends 
to have a narrow beam pattern which may, under 
some circumstances, be undesirable. It is necessary 
to have a pattern that directs sufficient energy off 
the axis to reach the desired targets. Hence, it may 
be desirable either to use a small projector or to 
direct the beam so as to scan the desired solid angle. 
In this connection, systems in which energy is radi- 
ated first to one side and then to the other side of 
the axis have been suggested by the British. If ar- 
rangements are made so that the torpedo itself 
moves in a circle, the beam scans a large region even 
though it itself is narrow. The turning must be at a 
rate slow enough so that the beam does not sweep 
over the target between pulses. Such a method is 
used in the ExFER42 mine. However, in case the 
torpedo starts out on a gyro-controlled course, the 
beam pattern must be wide enough to reach the 
target unless a searching procedure is introduced at 
a predetermined range. 

If the beam pattern is specified in advance, the 
wavelength and the linear dimensions of the hydro- 
phone must be kept proportional to each other. 
Hence, since the power that can be radiated is pro- 
portional to the area, the level that can be produced 
at the target increases with wavelength. To get a 
high signal level a large projector and a correspond- 
ingly long wavelength should be used. In case the 
pattern is not specified, the highest level is produced 
by the narrowest pattern. This implies a large pro- 
jector but a short wavelength. 

Another practical limit on the radiated power is 
the size and weight of the apparatus necessary to 


50 


SIGNAL AND NOISE LEVELS 


51 


drive the projector. Projectors are rarely more than 
50 per cent efficient and sometimes are as low as 20 
per cent so that from two to five times the power 
radiated must be supplied. If a very short pulse 
length is used, this power can be obtained from a 
storage condenser. so as to keep the average rate at 
which electric power is supplied relatively low. 

The important thing about the returned echo is 
not particularly its total energy but its spectrum 
level. If the energy is contained in a very narrow 
band of frequencies, and a sharply tuned receiver is 
used, the signal-to-noise ratio can be made higher 
than otherwise. However, if a short ping is used, the 
frequency band cannot be made too narrow. With a 
suitable definition of the band width Av and ping 
length r, it follows from Fourier analysis that for a 
suitably shaped pulse, 

1 

Av = -» 

T 

for r = 0.003 sec, Av = 333 c. This is a minimum, 
and for a ping with a square envelope the required 
frequency spread is greater. Furthermore, allowance 
must be made for the doppler effect because of both 
the motion of the torpedo and that of the target. 
Since the band width of the receiver must be filled 
by the echo to utilize the discrimination against 
noise, the band width of the projector must cover 
this, in addition to the expected doppler displace- 
ment. For this reason, the radiated band width has 
been usually selected of the order of magnitude of 
1,500 c. 

Let L t (d,R ) be the spectrum level of the radiated 
signal at a distance R from the projector and off the 
axis by an angle 6. It is assumed that the radiation 
pattern has circular symmetry so that 0 is the only 
angle on which it depends. It is also assumed that 
the intensity falls off with the square of the radius 
and with an attenuation factor /z. 

L t (0,R) = Ltifi) - 20 log R - nR (1) 
where L<(0) is the effective level at 1-meter distance 
and R is the distance in meters. The attenuation 
factor is subject to variation over rather wide ranges 
under different oceanographic conditions. In addi- 
tion, it increases with the frequency. In the neigh- 
borhood of 24 kc, 0.004 db per meter is a rough value 
for good sound conditions, while at about 60 kc it is 
near 0.014. If then Av is the effective band width 
over which the energy may be assumed to be uni- 
formly spread, 

P = 2tt X 6.45 X 10- 9 Av f; lO Lt(0)/lQ sin Odd. (2) 


The integral expresses the fact that the power is ra- 
diated in different directions and that the total power 
is the integral of the flow. The factor 6.45 X 10~ 9 
converts the level based on a root-mean-square pres- 
sure of 1 dyne per sq cm to power in watts per square 
meter. The absorption between the projector and a 
distance of 1 meter is neglected. The integral can be 
expressed in terms of the directivity index D and the 
level for 0 = 0 and R = 1, L t . Then it follows that 

L] = 10 log P - D — 10 log Av + 70.9. (3) 

The level at any other angle can be obtained by 
subtracting the difference L t — L t (0), obtained from 
the directional pattern. 

In addition to the strength of the signal, the 
strength of the echo depends upon the nature of the 
target and the way it reflects the signal. For many 
purposes, the effective reflecting power of the target 
can well be expressed in terms of the radius, a, in 
meters, of an equivalent sphere, or the target strength 

T = 20 log (4) 

This implies that the ship does not reflect like a plane 
but scatters energy in all directions, and serves as a 
satisfactory statement of the situation except when 
the torpedo is close to the target. A ship will have 
an effective target strength that depends strongly 
on its aspect. From ahead or astern the target 
strength will be much smaller than from abeam. 
Target strengths from zero to 25 db have been ob- 
served on surface ships. 21 For large submarines the 
target strength is about 25 db within 15 degrees of the 
beam. At other aspects it is rather variable because 
of the complicated shape of the submarine, but the 
values tend to fall around an average of 13 db. 

The significance of the target strength is that, 
added to the signal level at the target, it gives the 
level of the reflected pulse at a distance of 1 meter 
from the target. The level of the echo at the pro- 
jector is then obtained by subtracting another 
20 log R + fxR. Hence, if L° r is the reflected level, 

L° r = 10 log P — D — 10 log + 70.9 + T 

— 40 log R — 2^R. (5) 

As was indicated in the section on torpedo self 
noise, the level of the reflected signal must be at 
least equal to the hydrophone response to the back- 
ground and self noise. This sets an upper limit to the 
range that can be attained and a lower limit to the 


52 


SIGNAL AND NOISE LEVELS 


power that must be used for a desired range. The 
question of reverberation is not involved in the de- 
termination of the necessary power, since the 



Figure 1. Curves A, B, and C represent the signal 
level reflected from a target of strength 10 db as a 
function of the range in meters. All three curves cor- 
respond to a frequency spread of 1,500 c. Curves A and 
B represent an absorption coefficient of 0.004 db per 
meter and curve C one of 0.028 db per meter. For curve 
A the power is taken as 1,000 watts and for curves B 
and C as 100 watts. The directive indexes for curves 
A and C are —19 db and for B, — 11 db. Curves D, E , 
and F represent the noise level as a function of range 
from three types of ships, and curves G, H, and I rep- 
resent three possible torpedo background noise levels. 

reverberation is proportional to the power radiated. 

In Figure 1 curves A, B, and C show the level of 
the returned echo as a function of the range of the 
target for a number of assumed conditions. Curve A 
represents more or less the situation contemplated 


in a scheme for echo-ranging control of a large tor- 
pedo. The power P is 1,000 watts, the directive index 
D is taken as — 19 db, the frequency range as 1,500 
c, the target strength 10 db, and the absorption co- 
efficient n = 0.004 db per meter corresponding to a 
frequency near 25 kc. Such a torpedo might have a 
self-noise level of at least — 30 dbs so that the maxi- 
mum range to be expected would be in the neighbor- 
hood of 1,000 m. 

Curve B represents somewhat the possibilities in 
the case of the proposed Bowler method of control. 
For illustration, the power is taken as 100 watts, D 
as -11 db, Av = 1,500, T = 10 db and M = 0.004 
db per meter. In this case the self noise may still be 
expected to be near — 30 db so that the maximum 
range might be near 450 yd. 

Curve C corresponds to P = 100 watts, D = — 19 
db, = 1,500, T = 10, and ju = 0.028 db per 
meter. This is somewhat like the proposed ExFER42 
mine control. The absorption coefficient corresponds 
roughly to what might be expected at 60 kc and, as 
can be seen from the curve, this is important at 
ranges over some 400 yd. For short ranges, and par- 
ticularly for low frequencies, the absorption can 
usually be neglected. 

The self-noise level of the ExFER42 mine at 60 kc 
is not well established but may be as low as — 65 db. 
Hence, the possible range is 1,100 m even with this 
rather low power. 

In some cases the noise made by the target ship 
may be an effective part of the background noise and 
may tend to mask the echo. It may be possible to 
design the hydrophone and circuit so that the tor- 
pedo will steer on such noise, in which case the 
masking is unimportant. If, however, as is more 
usual, such a noise merely reduces the gain, it will 
be necessary for the echo level to be above the target 
noise as well as above the self noise. Figure 1 also 
shows curves D, E, and F of the expected target 
noise in several cases. From these it is clear that 
when the target noise exceeds the self noise of the 
torpedo, it is the target noise that sets the limit 
to the range that can be reached by echo ranging. 
Outside the useful range for listening, the echo- 
ranging system can be operated if enough power can 
be used. 


Chapter 12 

IDENTIFICATION OF THE ECHO 


O ne of the major problems in the design of an 
echo-ranging type of homing control is to pro- 
vide for suitable identification of the desired echo. 
The usual echo-ranging systems for locating sub- 
marines make extensive use of the skill of the opera- 
tor in distinguishing the desired echo from all the 
other noises that are present. Such skill is usually 
developed only after long practice, and to make such 
distinctions automatically is quite a different matter. 
It is one of the major problems in the design of this 
form of homing device. After the signal is sent out, 
the listening hydrophone will hear a number of 
different types of noise in addition to the desired 
echo from the target. These may be listed as: 

1. Self noise and background noise. 

2. Noise originating at the target. 

3. Reverberation. 

4. Bottom and surface echoes as well as echoes 
from extraneous objects. 

A number of schemes and devices have been sug- 
gested and discussed, and some of them have been 
tried, for distinguishing the desired echo from the 
other four types of noise. Several of these will 
be briefly indicated in this chapter and their per- 
formance with reference to the various interfering 
noises estimated. It is frequently possible to combine 
two or more of these schemes. 

12.1 USE OF A SHORT SIGNAL 

One method of identifying an echo is based on the 
use of a short pulse so that the returning echo con- 
stitutes a sudden and very short-lived increase in the 
intensity of the sound returning to the hydrophone. 
This method requires that the automatic volume con- 
trol [A VC] in the detecting circuit have a long time 
constant so that the amplifier gain will not be 
changed by this very short pulse. On the other hand, 
it is also necessary that the steering circuit have a 
very short time constant in order to respond to this 
sharp peak. With this system, any sound that per- 
sists much longer than a pulse and for a long enough 
time to operate the A VC circuit merely causes a re- 
duction in amplifier gain and does not affect the 
steering mechanism. 


This system has some disadvantages with respect 
to discrimination against the self noise and back- 
ground noise, since it may respond to sharp peaks in 
that noise because the time constant of the steering 
circuit is not long enough to smooth out these peaks. 
If, for instance, the self noise should be highly modu- 
lated, it might be necessary to set the amplifier gain 
so that the system would steer only on signals that 
are very much higher than the rms value of the 
background and self noise. The same thing is true 
with respect to noise originating at the target, so 
that such a system may have a disadvantage of as 
much as 6 to 10 db in its discrimination against noise 
originating at the target. 

On the other hand, this system is particularly good 
for discrimination against reverberation. Since the 
intensity of reverberation is proportional to the 
length of the pulse emitted, the reverberation will be 
of a low level because of the use of a short pulse. In 
addition, in so far as the reverberation is not too 
irregular in form and does not contain too many 
peaks, the A VC circuit will reduce the gain so as to 
discriminate against it. Since bottom and surface 
echoes are roughly of the same nature as reverbera- 
tion, the same statement applies to them. In general, 
it may be expected that a system using a very short 
pulse will not be limited by reverberation and bottom 
and surface echoes, but rather by self noise and noise 
originating at the target. The experience of the Bell 
Telephone Laboratories [BTL] in the application of 
echo-ranging homing and in the application of the 
Bowler scheme appears to bear out this expectation. 

The advantages to be gained by this method are 
somewhat tempered, however, by the fact that the 
sharpness of the emitted signal tends to make the 
reverberation consist of many sharp peaks. The 
study of reverberation has shown this difference in 
character when produced by short signals as com- 
pared with the smoother form produced by a long 
drawn out ping. Furthermore, since the echo is not 
returned from a simple plane surface, it tends to be 
drawn out to some extent. If the reflection were 
produced along the whole length of a 300-ft ship, 
bow on, the echo would be drawn out to 120 msec. 
Nothing so extreme as this is likely to occur, but a 




53 


54 


IDENTIFICATION OF THE ECHO 


3-msec signal will be returned as an appreciably 
longer echo if the effective reflecting surfaces are 
distributed over 10 ft or more in range. 

12.2 USE OF A LONG SIGNAL 

In distinction to the system described above, one 
may undertake to make use of a relatively long 
pulse. The development of the ExFER42 mine by 
the General Electric Company and the Leeds and 
Northrup Company, Inc. has proceeded along this 
line. This involves the use of some type of volume 
control that will not operate on this pulse. In the 
case of the ExFER42 mine, the pulse is of the order 
of 30 msec in length, and it is then necessary either 
to have an A VC that does not operate on a pulse of 
this length or else to have some other form of gain 
control in the amplifier. 

Because of the long time constants involved, this 
method may permit operation at only slightly above 
the rms value of the self noise and the background 
noise. Peaks in this noise will not affect the steering 
device at all. The same thing is true of noise originat- 
ing at the target. On the other hand, because of the 
long pulse, the reverberation will be correspondingly 
high. Bottom and surface echoes will also be at a 
high level, since their intensity is similarly more or 
less proportional to the pulse length used. Hence one 
may predict that systems using this scheme will be 
limited by reverberation, including bottom and sur- 
face reverberation, rather than by self noise. The 
experience of the General Electric Company with the 
ExFER42 mine appears to bear out this prediction, 
although of course the self-noise level of the body 
with which they were working was very low. 

12.3 THE USE OF A MODULATED 

SIGNAL 

One method that has been suggested for distin- 
guishing the echo from other noise is to impress on 
the outgoing signal a certain characteristic that can 
be recognized when the sound is returned as an echo. 
It is in this way that ordinary echoes are recognized 
in air when one recognizes a returning shout. The 
distinctive character of the sound may be in the 
form of simple modulation or it may involve fre- 
quency modulation. In fact, the two methods de- 
scribed above — that of a very short pulse and that 
of a very long pulse — may well be regarded as 
special cases of such modulation. 


It is normally expected that the character of the 
echo will not be present in the self noise and back- 
ground noise, since this noise is more or less random 
and if it has a modulation, the modulation of the 
signal can be selected to be distinctively different 
from it. The same thing is true, of course, of noise 
originating at the target except that its modulation 
may not be so well known in advance. Since the re- 
verberation and the echoes from the bottom and 
surface are returned from a great number of dif- 
ferent scatterers and different reflecting surfaces, the 
distinctive character of the pulse will be largely lost 
in them. On the other hand, it might be expected 
that the echo from a well-defined surface, such as 
that of a ship, might still contain a good deal of the 
character originally impressed upon the outgoing 
signal. This will probably be true to some extent, 
but, on the other hand, it must be remembered that 
the echo is not returned in general from a plane sur- 
face, especially in the case of a submarine. The echo 
is probably returned from a variety of different 
places on the submarine, so that the distinctive 
character of the signal will be preserved only in case 
it consists of modulation carried out so slowly that 
the change in character of the signal while passing 
over the target is not significant. Only a very small 
amount of experimental work on this point has been 
done in connection with homing devices. Somewhat 
more has been done with ordinary echo-ranging 
systems. It appears that the General Electric Com- 
pany in their work on the ExFER42 mine have in- 
vestigated a simple frequency sweep throughout 
their long pulse. They have not, however, used a 
detecting apparatus that was responsive only to a 
changing frequency. 

12.4 USE OF THE DOPPLER EFFECT 

Extensive experimental work has been done by the 
Harvard Underwater Sound Laboratory [HUSL] on 
the use of Doppler discrimination for identifying the 
desired echo. The signal returning from a reflecting 
surface will, in general, have a different frequency 
from that emitted because of the motion of the tor- 
pedo itself. This, however, is of little assistance be- 
cause of the fact that there will be self noise and 
background noise as well as noise from the target at 
all frequencies, and, consequently, there will be in- 
terference at the frequency of the returned echo. 
Similarly, the reverberation and the bottom and 
surface echoes will be shifted in frequency from that 


SIMPLE TIME VARIABLE GAIN 


55 


of the emitted pulse. If the target ship is at rest, the 
frequency of the echo will be shifted in the same way 
as that of the reverberation, but if the target ship is 
moving, the frequency shift will be different from 
that of reverberation and can be used as a means of 
separation. It has been possible to build a circuit 
that would recognize and operate only on echoes 
from a moving target. The presence of some rever- 
beration can be used in order to determine the speed 
of the torpedo itself and the Doppler effect due to its 
own motion. It also requires that the beam used be 
not too wide, for otherwise the Doppler effect in the 
reverberation due to the sound sent directly ahead 
will be so different from that due to the sound 
emitted more to the side that the proper correction 
for the speed of the torpedo cannot be made. To 
make the most of this system a long pulse must be 
used. As indicated above, a long pulse permits the 
most satisfactory discrimination against the self 
noise. Since the Doppler effect is used to discrim- 
inate against the reverberation the long pulse must 
be used to reduce the significance of the other limit- 
ing factor. 

The functional operation of such a scheme is 
moderately complicated, but can be made to operate 
more or less satisfactorily. It is, however, subject to 
the disadvantage that it cannot be made to operate 
against a stationary target. For details and discus- 
sion of this method, reference should be made to the 
work of the HUSL on Project NO-181. 

12.5 SIMPLE TIME VARIABLE GAIN 

To distinguish the echo from self noise and back- 
ground noise on the basis of intensity, it is only 
necessary to require a signal somewhat higher than 


the background noise to operate the steering system. 
If, however, it is required that a signal higher than 
the maximum of the reverberation is necessary for 
operation, the homing range will be almost in- 
finitesimal because of the extreme change in level 
of the reverberation. At some time shortly after the 
signal is emitted, the reverberation will reach a max- 
imum and then will fall off quite rapidly. In order to 
make use of signals of moderate level, it is necessary 
that the amplifier gain be increased as the rever- 
beration decreases, but that the increase be stopped 
at such a point that the system will not steer on self 
noise. This can be done by means of an A VC with a 
suitable time constant. 

For certain localities it may be possible to predict 
approximately the reverberation, and its rate of 
change, and to provide a simple change of the am- 
plifier gain that will compensate for this assumed 
reverberation. If this can be done, one has a rela- 
tively simple system of identifying the echo merely 
by means of its intensity. This is generally done, 
however, at the sacrifice of a certain amount of pos- 
sible homing range, because allowance must be made 
for the highest possible reverberation likely to be 
met. 

The above has been a very brief discussion of 
some of the methods that may be used to distinguish 
the desired echo from the undesired noises. This is 
the basic problem of echo ranging, and in the appli- 
cation of echo ranging and homing control use must 
be made of all of the knowledge gained in the study 
of echo ranging or sonar systems. Although con- 
siderable attention has been given to the problem, 
and it appears that the ExFER42 mine will operate 
at least in a moderately satisfactory way, a great 
deal of study is necessary before the best possible 
method of operation can be selected. 


Chapter 13 

APPLICATION OF ECHO TO TORPEDO STEERING 


T he problems of maneuverability and stability 
on course are somewhat different in the case of 
torpedoes that home by echoranging than in the 
case of those that home by listening. This is because 
the echo information on which the steering is based 
comes only at intervals, and the intervals are longer 
the longer the contemplated homing range. For ex- 
ample, if signals are emitted every second, the maxi- 
mum homing range is half the distance traveled by 
sound in a second, or roughly some 800 yd. 

The above restriction assumes that time must be 
given for the echo to return before another pulse is 
emitted, and can be modified if two or more pulses 
of widely different frequencies are used in sequence. 
A period of 1 sec could be allowed for the return of 
each frequency, but the number of signals returning 
per second would be equal to the number of different 
frequencies used. Pushed to the limit this would re- 
sult in an echo-ranging system similar to f-m sonar, 
and such a system might be useful in some cases. It 
seems questionable, however, whether such com- 
plication would ordinarily be justified. This is 
partly because the effective homing range that can 
be obtained with satisfactory reliability is perhaps 
not over 800 to 1,000 yd because of water conditions. 
When the ranges are limited to this amount, infor- 
mation can be obtained every second and this is 
probably sufficient for steering at a target whose 
maneuverability is similar to that of ships in use 
today. 

While far from being a comprehensive study of the 
problem, three methods have been given a prelimi- 
nary trial. All of these are subject to the restriction 
just discussed. 

13.1 THE BOWLER SCHEME 

This is fundamentally the simplest method. It was 
suggested and started by the British, and was taken 
up and studied for a time by Bell Telephone Labora- 
tories, Inc. [BTL]. 

The Bowler scheme is based on the idea of making 
the torpedo go into a circle when in the neighborhood 
of the target, and the echo-ranging system is used 
essentially to determine when the torpedo is in the 


neighborhood of the target, and to which side of its 
first course it should circle. In the model later 
studied by BTL a hydrophone was placed on each 
side of the head of a Mark 13 torpedo. Sound pulses 
were emitted from both sides and the presence of an 
echo could be detected in one or the other hydro- 
phone. In fact, the two sides are essentially inde- 
pendent. Each side is charged with the responsi- 
bility of determining if there is a target on its side. 
A satisfactory echo from one side puts the rudder 
hard over to that side and keeps it there so that the 
torpedo circles toward the target. 

Preliminary analysis indicated that this simple 
scheme should very much increase the probability of 
an effective bow shot, while it would leave the proba- 
bility of a hit from the beam essentially unchanged. 
It might, in fact, provide a target of roughly the 
same effective width from all directions. On the 
other hand it is clear that a homing range much 
greater than the turning radius of the torpedo cannot 
be used, and that both of these must be kept less 
than the length of the ship for a bow shot to be ef- 
fective. The selection of the homing range and the 
turning radius must be made in the light of some 
knowledge of the length of the ship to be attacked, 
the relative speeds of torpedo and its target, and the 
relative importance of bow and stern shots. 

In this system everything depends upon the proper 
identification of a single echo, for a false echo on the 
wrong side would turn the torpedo away from the 
target and it would never return. A false echo on the 
correct side might still cause the torpedo to turn 
toward the target, but to turn too soon and hence to 
miss. To minimize the possibility of steering on false 
echoes, it has been suggested that two or three suc- 
cessive echoes be required to put the rudder over. 

With respect to steering problems this is perhaps 
the simplest possible system. It is subject, however, 
to all of the difficulties associated with identification 
of the echo that were described in the previous chap- 
ter; and this problem is of extreme importance for 
the reason just indicated. The difficulty is somewhat 
minimized, however, by the fact that only short 
homing ranges, of the order of 100 yd, are desired. 


56 


CORRECTION OF GYRO COURSE 


57 


As a steering system the Bowler idea is unques- 
tionably satisfactory, since a torpedo can easily be 
made to run in a circle at a set depth. It only remains 
to be determined whether the changeover from a 
straight course to a circle is an effective maneuver, 
and whether the echo-ranging system can identify 
the desired echo in order to make the switch with 
sufficient reliability and at the right time. Up to the 
present this has not been given any extensive field 
tests. 

13.2 METHOD OF THE ExFER42 MINE 

In the ExFER42 mine, steering and searching are 
combined in a simple and effective manner by giving 
the mine left rudder when no echo is identified and 
right rudder when an echo is heard. In practice this 
is made a trifle more complicated to minimize the 
effect of false echoes but the principle is as just 
stated. In the absence of an echo the mine turns 
steadily in a circle and searches for a target. When 
an echo is received and identified, the rudder is 
thrown right and held there until a certain period 
elapses during which no echo is heard. By this means 
the mine is made to approach the target along a 
sinuous path. 

Here, as in the Bowler scheme, the rudder is 
thrown hard over and the mine goes into a circular 
path. It is important that the angular rate of turning 
be so related to the beamwidth of the projector and 
receiver that the beam does not sweep across the 
target before there is sufficient time to get a good 
echo. 

Roughly, it may be said that the mine steers at 
one end of the target and the wake, since the wake is 
also a target which reflects the sound. As just de- 
scribed, the mine steers at the right end of the com- 
bination. If the normal direction of circling, without 
echo, is to the right, the mine will tend to steer at 
the left end. This may have some advantage in per- 
mitting the mine to follow along a wake until it 
comes to the target ship, unless of course it is going 
in the wrong direction. Preliminary field tests have 
shown that this system will operate satisfactorily 
against a submarine when it is combined with a suit- 
able system for steering in depth. Presumably it will 


also operate against a surface vessel but extensive 
tests on this point have not been carried out. 

This introduces more dynamic problems than does 
the Bowler scheme because the torpedo is not al- 
lowed to settle down into a steady turning circle but 
may be barely out of the transient state before the 
rudder is reversed. In designing the system it is im- 
portant to know the duration of this transient state. 
It is also important to minimize the roll both during 
the transient and the permanent state of turning. 
This is especially true if the projector is not sym- 
metrical about the torpedo axis. 

13.3 CORRECTION OF GYRO COURSE 

The method that has been proposed by BTL for 
echo-ranging control of the Mark 14 and other tor- 
pedoes applies the acoustic information to the cor- 
rection of the gyroscope heading. This can be done 
rather easily through the gyro preset mechanism, 
and the questions of stability on course are then 
merely those of the ordinary gyroscope steering. 

In this system as experimentally built up, the re- 
ceiving system determines the bearing of the target 
that returns the echo and turns the gyro to this 
bearing as long as it is less than some 6 or 7 degrees 
away from the torpedo axis. If, however, the target 
is farther than this from the torpedo axis, the gyro 
is turned only a maximum amount of some 6 or 7 
degrees. This maximum angle is approximately the 
angle through which the torpedo itself can turn in 
the interval between successive echoes. 

A similar method is used in the depth control of 
the ExFER42 mine. In this system the echo is in- 
terpreted as coming from either above or below the 
torpedo axis, and a pendulum that controls the rate 
of dive is adjusted accordingly. 

Both of these applications make use of the infor- 
mation derived from the echo to make adjustments 
of the normal torpedo control mechanism. If these 
adjustments are made slowly enough, the dynamic 
properties and the stability of the control system are 
not affected. This is the occasion for the limited rate 
of turn indicated above in the Mark 14 experimental 
control. On the other hand, the adjustments must 
be made rapidly enough to provide the desired hom- 
ing on the target. 


Chapter 14 

NEEDS FOR FURTHER STUDY 


T he above outline indicates in a rough way the 
general state of knowledge concerning acoustic 
homing torpedoes. It is quite clear that the present 
knowledge of the subject is very sketchy, and is only 
barely sufficient to permit the construction of usable 
weapons. Many of the gaps in understanding have 
been quite apparent in the previous chapters but 
this chapter will constitute a brief summary of some 
of the principal lines along which additional research 
needs to be carried out. 

14.1 TACTICAL ANALYSIS 

A good deal of study is needed to make clear what 
kinds of homing weapons are desirable and needed. 
It is easy to say that a homing torpedo should run 
as fast as possible, travel as far as possible, and 
should have as great a homing range as possible. In 
addition it should be as light as possible and carry 
as heavy an explosive charge as possible. It is ob- 
vious, however, that all of these ends cannot be ac- 
complished at the same time. Some of them are more 
or less opposing, and it is important to make a care- 
ful evaluation of each one so that it can be properly 
appraised in reference to the others. Such a study 
would provide the basis for designing a suitable 
compromise. 

A useful tactical analysis must be based on in- 
telligent estimates of the objectives to be desired. 
It is probably not very practical to have a universal 
weapon which would naturally not be ideally adapted 
to any one purpose. A torpedo for use against mer- 
chant vessels may be quite different from one for 
use against naval vessels. The former may require 
only a slow running speed but possibly a long under- 
water range. It need have only a moderate explosive 
charge but may possibly require a long homing 
range. A torpedo for use against warships may re- 
quire a much higher explosive charge, will certainly 
require a higher underwater running speed, and it 
may be proper to attain this latter by sacrificing 
homing range as well as underwater range. A torpedo 
to be launched from aircraft should probably have 
quite different specifications from one to be launched 
from submarines because the element of stealth is 
practically lacking from its tactical use. 


Considerable attention also needs to be given to 
the subject of decoys. It seems probable that any 
acoustic homing torpedo is subject to a decoy of 
some kind, but careful analysis of the nature of pos- 
sible decoys and their probable modes of operation 
can lead to a specification of a homing device of such 
a nature that the decoy problem is made as difficult 
as possible. 

These tactical problems have already been given 
some attention, 22 and some tentative conclusions 
have been reached, but much more work on the 
problem is necessary before it can be considered as 
fully understood. 

14.2 SELF NOISE STUDIES 

The self -noise problem is really the heart of the 
acoustic homing torpedo problem. As has already 
been indicated, a great deal of work still remains to 
be done before the sources of noise are understood 
and before the best method of reducing and dis- 
criminating against this noise can be specified. The 
studies necessary in this connection can be grouped 
under several headings. 

1. A thorough study of propeller cavitation is 
needed to determine if this source of noise can be 
eliminated while at the same time adequate thrust 
is maintained. Up to the present practically nothing 
has been put into practice along this line, and cavi- 
tation noise is reduced only by running the torpedoes 
at considerable depths. Against surface ships this 
makeshift involves all of the difficulties of acoustic 
steering in the vertical plane. These could be elimi- 
nated if quiet propellers were available. 

Such a study will presumably require a carefully 
coordinated combination of theoretical and experi- 
mental work. This type of thing has been started by 
the Harvard Underwater Sound Laboratory, but it 
is a long-time program and it is probably only after 
several years of work that practical results can be 
expected. 

Cavitation at other points on the torpedo must 
also be avoided but this is apparently not trouble- 
some until speeds much higher than those at which 
the propeller cavitation now provides a dominant 
source of noise. 


58 


ELECTRIC CIRCUIT AND CONTROL METHODS 


59 


2. Attention is also needed to methods of reducing 
the machinery noise inside the torpedo and of the 
transmission of this noise to the shell. The sources of 
the noise are not at all clearly understood, and the 
methods of acoustically isolating the machinery from 
the shell have only been given very preliminary con- 
sideration. 

3. The question of the dependence of self noise on 
frequency has not yet been given more than a very 
hasty examination. It is not known whether the use 
of higher frequencies would decrease or increase the 
difficulty due to cavitation noise nor how it would 
affect the problem of machinery noise. 

4. A great deal of work is yet to be done on the 
problem of hydrophone isolation and the best way 
to provide a hydrophone that discriminates against 
the self noise of the torpedo. Presumably different 
characteristics are required to discriminate against 
cavitation and against machinery noise. Presumably 
the directivity pattern is of some importance, but 
the method of mounting is certainly of equal im- 
portance. 

14.3 HYDROPHONE STUDIES 

In addition to the design of hydrophones to dis- 
criminate against self noise, considerable work seems 


called for on the general subject of the proper kind of 
hydrophone to use under the severe conditions to 
which a torpedo is subject. Various types of crystal 
and magnetostriction hydrophones have been tried 
and suggested. Some of them have operated through 
the torpedo shell and some have required a hole cut 
in the shell. All of them are more subject to damage 
than would be desirable. 

14.4 ELECTRIC CIRCUIT AND 
CONTROL METHODS 

The techniques available in electric circuits and 
electrical and mechanical methods of applying the 
acoustical information to the steering of the torpedo 
seem adequate to meet the requirements. Of course 
considerable work can be done to improve the re- 
liability and simplicity of the whole system but it 
seems more important to determine what it is de- 
sired for the control system to do. This must be 
based on the kind of study included under tactical 
analysis. 

Of course, additional study is always called for to 
improve methods of manufacture and maintenence, 
but the subjects just indicated seem to be those on 
which study is necessary to improve the functional 
performance of the homing torpedoes. 























































PART II 


ECHO-RANGING TORPEDO CONTROL SYSTEMS 









Chapter 15 

INTRODUCTION 


A n echo-ranging torpedo control system is one 
which employs a transmitter, operating at a 
definite frequency, which sends out an acoustic pulse 
into the water at periodic intervals and then controls 
on the echo, which is reflected back from the target. 

Since the hydrophone in an echo-ranging control 
system is exposed to the self noise of the torpedo 
while it is listening for the returning echo, the re- 
sponse of the hydrophone to the self noise deter- 
mines the lowest level of echo which can be effective 
in control of the torpedo. The following review of 
terminology and units should help the reader to fol- 
low later discussions of problems involving the use of 
transducers. 

When a hydrophone is exposed to a pure-tone 
acoustic signal in water, a voltage is developed across 
its terminals because of the dynamic sound pressure 
on its surface. It is common practice in acoustics to 
express power and voltage ratios on a logarithmic 
scale. If there are two values of power, Pi and P 2 , 
the result is by definition, 


p i 

10 log — = power ratio 
P2 


expressed in decibels (db). If the two amounts of 
power are generated in systems of the same im- 
pedance, voltages will be developed such that the 
power is proportional to V 2 . Then, 


Vi 

20 log — = corresponding voltage ratio in db. 
V2 


If V 2 is 1 v, V 1 can be expressed as 20 log V\ decibels 
relative to 1 v. The sensitivity of a hydrophone is 
normally expressed as the number of decibels rela- 
tive to 1 volt per dyne per sq cm rms dynamic sound 
pressure when a pure-tone signal is used. 

If the hydrophone is used to measure random 
noise in water, a reference pressure of 0.000204 dyne 
per sq cm is used, and the noise sensitivity of the 
hydrophone is expressed as the number of decibels 
relative to 1 volt per 0.000204 dyne per sq cm per c. 

The intensity of a sound field is expressed in terms 
of the pressure generated in a hydrophone in a 1-c 
band width. Arbitrarily, a sound field is defined as 
zero spectrum level sound field when it is capable of 


generating a dynamic sound pressure of 0.000204 
dyne per sq cm in a band width of 1 c. 

The intensity of the sound field is expressed in 
decibels spectrum (dbs) equals 20 log (rms dynamic 
sound pressure per cycle)/ (0.000204 dyne per sq cm 
per c). Since the power passing through unit area in 
the sound field is proportional to the square of the 
pressure, pressure ratios in the sound field can be 
expressed on a decibel scale as 

p 

pressure ratio in db = 20 log — • 

P 2 


A zero spectrum level sound field therefore generates 
a pressure of 74 db below 1 dyne per sq cm per c. 

If the band width to which a hydrophone and its 
associated equipment is sensitive is considered, the 
power absorbed by the hydrophone will be propor- 
tional to the band width in cycles per second. When 
two frequency ranges A/i and A f 2 are considered, the 
ratio of equivalent power expressed in decibels is 


10 log 


A/i 

A/2 


The voltage generated in a hydrophone is equal to 
hydrophone sensitivity + 20 log p/0.000204 + 10 
log (band width in cycles per sec)/(l c). The hydro- 
phone sensitivity is the number of decibels relative 
to 1 volt generated by a sound field of zero spectrum 
level when the band width is 1 c. The rms sound 
pressure is p in the field in a 1-c band width and 
20 log p/0.000204 is equal to the intensity of the 
sound field expressed in dbs. 

As a practical example consider a noise field of 10 
dbs which is being measured with a given transducer. 
Assuming that this signal is fed into two different 
receivers, one with a band width of 4 kc and the 
other with a band width of 1.4 kc, in the first case, 
that of the signal level as seen by the receiver of a 
4-kc band width, the 10 dbs noise is 10 db above 
zero spectrum. The 4-kc band width is then 36 db 
above a 1-c band width and the effective dynamic 
sound pressure will be — 74 + 10 + 36 = — 28 db 
below 1 dyne per sq cm. If the sensitivity of the 
transducer is 94 below 1 v per dyne per sq cm per c, 
the effective voltage developed by the transducer for 


flHHi 


63 


64 


INTRODUCTION 


the 4-kc band -width receiver will be —28 — 94 = 
— 122 db below 1 volt. In the second case of the 1.4-kc 
band-width receiver, the effective dynamic sound 
pressure will be — 74 + 10 + 31.5 = —32.5 db be- 
low 1 dyne per sq cm and the effective voltage de- 
veloped on the transducer will be —32.5 — 94 = 
— 126.5 db below 1 v. 

Another important characteristic of a transducer 
is its directivity index. If a hydrophone is uniformly 
sensitive to sound incident from all directions, its 
directivity index expressed in decibels will be zero. 
If two hydrophones have the same sensitivity but 
different directivity index, they will give the same 
response to a unidirectional sound providing the 
sound arrives at both of the hydrophones on the 
axis of maximum sensitivity. If the hydrophones are 
placed in a sound field such as the ambient sound 
field in a body of water, where sound is incident uni- 
formly from all directions, then the level of signal 
recorded by Ihe two hydrophones will be different. 
For example, assume that two hydrophones, A and 
B, have the same sensitivity, but that A has a direc- 
tivity index of — 10 db while B has a directivity index 
of —20 db. Supposing that hydrophone A was used 
to measure ambient water noise with a given am- 
plifier and then hydrophone B is substituted, the 
signal level observed with B will be 10 — 20 = — 10 
db, or 10 db lower than the signal observed with 
hydrophone A. 

When a transducer is used as a projector it trans- 
forms electric energy from an alternating current 
into an alternating sound pressure; the relation be- 
tween power and rms dynamic sound pressure is 
analogous to the familiar relationship connecting 
electric power, voltage, and resistance, and is given by 



pc 


P is the power delivered measured in ergs per second, 
p is the rms dynamic sound pressure in dynes per 
square centimeter, pc is the radiation resistance 
where c is the velocity of sound measured in centi- 
meters per second, p is the density of the medium in 
grams per cubic centimeter, A is the area of surface 
in square centimeters through which the sound 
passes. For example, if a projector of directivity in- 
dex zero emits 400 watts of power into water, the 
rms pressure at 1 meter can be calculated from the 
preceding formula by use of the values 

pc for water = 150,000 gm per sq cm per sec, 

A for 1-meter distance = 126,000 sq cm, 


Since 


150,000 P 
126,000 


1.19 P. 


P = 400 watts = 40 X 10 8 ergs per sec, 
p = 40 X 1.19 X 10 4 = 6.9 X 10 4 dynes per sq cm, 
6.9 X 10 4 dyne per sq cm = 97 db above 1 dyne per 
sq cm. 

In case the directivity index of the transducer has 
the not unreasonable value of — 23 db, the rms sound 
pressure on the axis of the transducer at 1-meter dis- 
tance will be 


p = 97 + 23 = 120 db above 1 dyne per sq cm. 


If the sensitivity of a hydrophone is measured as 
a function of angle of incidence of the signal, the 
sensitivity normally varies in such a way that it 
reaches a maximum in one direction called the axis 
of the transducer. The curve showing the variation 
of sensitivity as a function of angle is called the pat- 
tern of the transducer. The directivity characteris- 
tics of the transducer are normally expressed in 
terms of the number of decibels reduction in sensi- 
tivity for a given angle measured from the axis. The 
curve of sensitivity as a function of angle usually 
shows secondary maxima of sensitivity at fairly large 
angles from the axis. These secondary maxima are 
called minor lobes and their sensitivity compared to 
the sensitivity on the axis of the transducer is an 
important consideration in the performance of the 
device. 

An echo-controlled torpedo must be able to send 
out an acoustic signal, receive the echo reflected 
from a target, and on the basis of the information 
supplied by the echo, steer toward the target. The 
factors which influence the effectiveness of the echo 
are self noise of the torpedo, thermal gradients in the 
water, strength of the target, reverberation, and the 
use of countermeasures by the enemy. 

The projector on the torpedo converts the electric 
energy generated in the transmitter into acoustic 
energy. The projectors normally used are designed 
with directive indexes of — 20 to — 25 db so that the 
acoustic energy is concentrated on the axis. As the 
sound progresses through the water it is reduced in 
intensity by the inverse square law and by absorp- 
tion in the water. When it strikes the target a certain 
percentage is reflected back and on the return path 
the losses due to the inverse square law and water 
absorption again take place. In addition, thermal 


SECRET 


INTRODUCTION 


65 


gradients in the water can cause a further loss in the 
echo intensity. 

During the period immediately following trans- 
mission of the acoustic pulse, reverberation is re- 
turned from the surrounding volume of the water 
and from the surface and bottom. This reverberation 
intensity decreases with time, and, although it is 
necessary to protect the receiver against it during 
the initial stages, it is not a factor which will affect 
maximum range. 

In order to consider the problem exactly it is neces- 
sary to define a quantity called the target strength. 
Since it is possible to compute the reflectivity of a 
perfectly reflecting sphere, the equivalent sphere size 
is a convenient quantity to associate with a target. 



10 20 30 40 50 70 100 


EQUIVALENT SPHERE DIAMETER IN FT 

Figure 1 . Relation between target strength and equiv- 
alent sphere size and representative target strengths. 

If the transmission and reflection process are formu- 
lated more exactly, another more convenient means 
of expressing target strength can be derived. 

Let L 0 = rms pressure level of a transmitted pulse in 
db vs 1 dyne per sq cm at 1 yd from the 
projector in the direction of the acoustic 
axis; 

R = range in yards to the target; 
a = attenuation in db per yard at the specified 
frequency. 

The pulse intensity at the target may be expressed as 
Lq — 20 log R — aR 

expressed in db vs 1 dyne per sq cm. 

At the target, a certain fraction of the energy is 
reflected and the reflected ray suffers the same loss in 
returning to the target as it did in going out to the 


target. The signal strength returned to the trans- 
ducer will then be 

Lq — 20 log R — aR — 20 log R — aR + T. 

Loss to target Loss from target 

In this expression T is the strength of the target in 
db necessary to make the above expression equal to 
the intensity of the returned echo. 

It has been shown that 

T = 20 log D - 12 

where D is the equivalent sphere diameter. A sphere 
of 4-yd diameter therefore has a strength T — 0. 

Figure 1 shows values of T for several types of 
ships and it also shows the relation between target 
strength T and equivalent sphere size. 

In order to utilize an echo for control of a torpedo 
it is necessary to have a small margin of signal level 
over the level of the self noise of the torpedo. It is 
possible to define the acoustic range of an echo- 
ranging torpedo by means of the following equation : 

L 0 - 40 log R - 2aR + T = S + 10 log W + M - 74 

where L 0 = the acoustic rms pressure level of the 
transmitted pulse in db vs 1 dyne per sq 
cm at 1 yd from the transducer on the 
acoustic axis; 

R = the acoustic range in yards; 
a = the attenuation in db per yard; 

T = the target strength in db; 

S = the rms self-noise level in db spectrum 
(vs 0.000204 dyne per sq cm per c) as 
measured with the torpedo transducer 
used as a receiver; 

W = the band width of the system in cycles 
per second; 

M = the signal-to-noise margin in db required 
by the system. 

If S is given in db vs 1 dyne per sq cm per c, the 
factor —74 should be omitted. 

Table 1 shows values of L 0 for a series of values of 
acoustic power and for two values of directivity in- 
dex. 

Table 1 


Acoustic power 
in watts 

L 0 in db vs 1 dyne/cm 2 at 1 yd 

For D = — 20 db For D = - 25 db 

250 

115.5 

120.5 

500 

118.5 

123.5 

1,000 

121.5 

126.5 

2,000 

124.5 

129.5 


SECRET 


66 


INTRODUCTION 


Two commonly used frequencies for echo-ranging 
control are about 25 and 60 kc. The value of D for 
the 25-kc systems is approximately —20 db while 
for the 60-kc systems it is approximately 25 db. 
Making the assumptions indicated in Table 2 which 
are consistent with the best information available it 


Present information indicates that the lowest feasi- 
ble values for the torpedo self noise in a 30- to 35- 
knot torpedo are 15 to 20 dbs at 25 kc and 5 to 10 
dbs at 60 kc. 

Echo-ranging torpedo control has been used in all 
torpedo applications. The actual design of the con- 



lOO 200 500 700 1000 2000 5000 


ACOUSTIC RANGE IN YARDS 

Figure 2. Acoustic range as a function of torpedo self noise at 25 kc. 


is possible to calculate values ol the maximum pos- 
sible acoustic range as a function of torpedo self noise 
for different values of acoustic power output. 


Table 2 


Quantity 

25 kc 

60 kc 

a 

0.003 db per yd 

0.015 db per yd 

T 

10.5 db 

10.5 db 

W 

1,500 c 

1,500 c 

M 

3 db 

3 db 


Figures 2 and 3 show the maximum ranges calcu- 
lated as a function of torpedo self noise for a series of 
acoustic powers for the 25-kc frequency and the 60-kc 
frequency. 

It must be remembered that these are the values 
obtained under ideal operating conditions and will 
be decreased by the additional attenuation produced 
by thermal gradients if they are present. 


trol system, however, is determined to some extent 
by the application in which it is to be used. In anti- 
submarine applications, the torpedo is launched 
either from an aircraft or from a surface ship. When 
launched from an aircraft, the actual position and 
bearing of the submarine are not known. The tor- 
pedo is normally dropped as near as possible to the 
swirl left by the diving submarine. In this case the 
torpedo normally searches in a circle either at some 
fixed depth or with the depth continuously increas- 
ing. As soon as the torpedo receives an echo from the 
target it goes into acoustic control in both azimuth 
and depth until it either strikes the target or loses 
the acoustic contact. It is obviously necessary in this 
type of device to use acoustic control in both azimuth 
and depth. 

In the case of a surface-ship-launched antisub- 
marine torpedo, there is some knowledge of the bear- 


INTRODUCTION 


67 


ing of the target at the time of launching, but the 
torpedo is launched at such short ranges that cir- 
cling search is employed. Since the amount of ex- 
plosive which is necessary to cripple a submerged 
submarine is less than that necessary to cripple a 
surface warship, a. small torpedo is usually used in 


limits the angle over which it is possible for the tor- 
pedo to locate a target in the initial search. Two 
compromises have been used. One is to make the 
beam pattern of the hydrophone very narrow in the 
vertical plane and broad enough to cover an angle of 
approximately 60 degrees in the horizontal plane. 


OJ 

O 

o 

o 

• 

o 


c n 
> 

CD 

O 


LU 

CO 

o 

z 

u_ 


UJ 

if) 


O 

O 

UJ 

Q. 

or 

o 


co 

cr 



100 200 500 700 1000 2000 5000 

ACOUSTIC RANGE IN YARDS 


Figure 3. Acoustic range as a function of torpedo self noise at 60 kc. 


this service. For tactical reasons, these torpedoes 
have been designed so that they can be carried in the 
bomb bays of ordinary bombing planes. 

In anti surface-ship applications the torpedo may 
be launched from a submarine, from another surface 
ship, or from an aircraft. In these applications the 
bearing of the target is quite accurately known at 
the time of launching, and the range from which the 
torpedo may be fired is considerable. Since the oper- 
ating range of these torpedoes is greater than their 
acoustic range, it is necessary to use gyro control in 
the initial portion of the run. From the standpoint 
of effects of self noise of the torpedo and the effec- 
tiveness of the countermeasures used against it, it is 
desirable to have hydrophones with quite sharp 
beam patterns. The effectiveness of the projector is 
also increased if its beam pattern is sharper. How- 
ever, making hydrophone beam patterns very sharp 


The other is to use a hydrophone with sharp beam 
pattern in both planes; but to control the gyro course 
so that, instead of running straight, the torpedo os- 
cillates back and forth over the direction of firing so 
that its hydrophone can scan an angle of 60 to 80 
degrees. Since the vertical level of a surface ship is 
fixed, anti surface-ship echo-ranging torpedo control 
systems are often arranged only for azimuth control 
and they depend on a purely hydrostatic control to 
determine the running depth. Since the self noise of 
a torpedo is determined largely by propeller cavita- 
tion, there is some advantage in operating at a depth 
of approximately 50 ft for the initial portion of an 
attack. In order to do this it is necessary to include 
means of acoustic control of depth in order to bring 
the torpedo up near enough to the surface to strike 
the target. 

An echo-ranging torpedo control system is neces- 



68 


INTRODUCTION 


sarily more complicated than a listening acoustic 
system. In order to justify the use of the more com- 
plicated system it must have sufficient advantage 
over the simple listening system. The echo-ranging 
device has the following advantages. 

The countermeasures which are required to pre- 
vent it from striking a target are ordinarily different 
from those which are used against listening torpedoes. 
A properly placed noisemaker which simulates the 
noise of a ship is sufficient to limit the effectiveness 
of the listening device. Since it is possible to control 
the nature of the transmitted pulses in an echo- 
ranging system it is possible to predetermine the 
kind of countermeasure which will be necessary in 
order to limit its effectiveness. Variation of such 
things as the frequency of the acoustic signal, or the 
length of the transmitted pulse can be used to in- 
crease the difficulty in the use of countermeasures. 
One of the most effective means of countering an 
ordinary listening torpedo is for the target to slow 
down so that the noise generated by its propellers is 
much reduced. This procedure would be ineffective 
against an echo-ranging device. In the case of use as 
an antisubmarine device, echo-ranging torpedoes 
have been made to follow the wake of the submarine 
and make successful attacks from ranges even greater 
than possible acoustic ranges. Since the energy in the 
transmitted signal is concentrated at a single fre- 
quency, the frequency range of sensitivity in the re- 
ceiver can be considerably less than that in the re- 
ceiver of a listening torpedo. This means that the 
performance of the echo-ranging system will be less 
limited by the self noise of the torpedo than is the 
case in the listening devices. It is not possible to fire 
listening torpedoes in salvos because the later ones 
fired will tend to follow the earlier ones. By design- 
ing echo-ranging torpedoes so that more than one 
operating frequency is used, it is possible to fire suc- 
cessive echo-ranging torpedoes by using different 
torpedoes whose systems operate on different fre- 
quencies. If the echo-ranging system is so designed 
that it will not steer on noise, salvo firing can even 
be used with only one frequency of operation, pro- 
viding the intervals of firing are great enough so that 
the sensitivity of the receivers of the later torpedoes 
will not be reduced because of the noise generated 
by the propellers of the first ones fired. A listening 
torpedo controls on the noise generated by the pro- 
pellers of the target. The tendency is, therefore, for 
the torpedo to strike at or near the propellers, which 
will cripple the target but not necessarily sink it. On 


the other hand, an echo-ranging torpedo tends to 
strike further toward the bow of the target and it is 
even possible to control to some extent the part of 
the target on which the torpedo will strike. 

The use of an echo-ranging system also has some 
disadvantages. Perhaps the most important is the 
fact that its use gives it away. Very shortly after 
firing, the transmitter starts sending out acoustic 
pulses which can be received by the sound gear on 
the target. This makes possible the more effective 
timing of the use of countermeasures, and when the 
torpedo is fired from a submarine, it tends to give 
away the approximate bearing of the submarine at 
the time of firing. The greater complication in the 
control system necessary in order to utilize the maxi- 
mum advantages of an echo-ranging system makes 
the maintenance and service problems with these 
torpedoes more difficult. The effectiveness of an echo- 
ranging control system also depends to a greater 
extent on external water conditions than is the case 
with a listening system. Since it is necessary for the 
acoustic signal to travel two ways instead of one, the 
conditions in the water unfavorable to the trans- 
mission of an acoustic signal will have a chance to 
operate on the echo-ranging signals twice. Layers 
causing refraction and reflection due to variations in 
salinity and temperature may sometimes cause con- 
siderable difficulty. Sometimes even masses of sea- 
weed or schools of fish will behave as fictitious 
targets. In order to determine whether an echo- 
ranging system should be used rather than a simple 
listening system it is necessary to weigh these vari- 
ous advantages and disadvantages. The most effec- 
tive arrangement is probably the use of both, with 
echo-ranging torpedoes to be used where the counter- 
measure problem is most important and listening 
torpedoes to be used where it is especially important 
to avoid detection of the submarine by the enemy. 
An acoustic torpedo provided with a switch on the 
outside of the body which could set it up either as a 
listening or an echo-ranging device, depending on the 
conditions existing at the time of launching, would be 
an ideal weapon. 

In order to operate an echo-ranging torpedo, it 
is necessary to transmit pulses of signal at intervals, 
in order to provide periods in which the receiver can 
listen for the returning echo. Since sound has a ve- 
locity of about 5,000 fps in water, it is necessary to 
provide a listening interval of 1 sec for each 2,500 ft 
of range. If the maximum possible range of the tor- 
pedo is increased, the interval between transmitted 


INTRODUCTION 


69 


pulses must be increased in proportion. For this 
reason, the frequency with which information for 
acoustic control of the torpedo is received becomes 
less as the maximum range is increased. 

The most common steering system used in tor- 
pedoes is the on-off type in which the rudder is 
turned either hard to port or starboard. If such an 
on-off steering system is used in a torpedo under 
echo-ranging control, the amplitude through which 
the torpedo will oscillate in the course of an attack 
will increase with the maximum acoustic range of the 
torpedo. If the transducers used have narrow beam 
patterns in azimuth, there is danger of information 
being received when the axis of the torpedo makes 
a considerable angle with the target bearing. It is 
necessary, for this reason, to consider very carefully 
the body dynamics of the torpedo and the sharpness 
of the beam pattern of the transducer in determining 
the maximum possible range which can be achieved 
using an on-off type of steering control. As the maxi- 
mum ranges of echo-ranging torpedoes are increased, 
it will probably become more and more important 
to develop proportional types of steering control. 

Assuming that the acoustic pulse which is trans- 
mitted into the water by a given transmitter and 
projector represents a given amount of acoustic 
power, the level of the echo returned by a target will 
be a function of the distance from the torpedo to the 
reflecting target. For greater distances between the 
torpedo and the target, the level of the echo will be 
less. Since the time of transit of the signal from the 
projector to the target and back to the hydrophone 
is a direct measure of the range of the target, it is 
possible to use a time variation in sensitivity of the 
receiver to take care of the variation in echo level 
with range. Another factor which makes this varia- 
tion of receiver sensitivity with range important is 
reverberation. When the pulse of acoustic energy is 
transmitted into the water, a signal is received by 
the hydrophone because of the sound energy scat- 
tered back from the surrounding volume of water. 
This sound is known as volume reverberation. In 
addition to the volume reverberation, some sound is 
scattered from the surface of the water and also from 
the bottom of the ocean. These signals are known re- 
spectively as surface and bottom reverberation. Their 
importance relative to the volume reverberation is 
determined by the nearness of the surface and the 
bottom to the torpedo. In most cases the surface or 
bottom reverberation is the most important factor 
in the total reverberation level. The level of rever- 
beration decreases with time after transmission in 


much the same manner as the echo level from a given 
target. Use of time variation of sensitivity of the re- 
ceiver can be used to prevent it effectively from caus- 
ing the torpedo to steer on bursts of reverberation. 
This time variation in gain of the receiver amplifier 
can be controlled by the same time base which is used 
to control the time of transmission. This type of gain 
control is usually designated as TVG. In operating 
torpedoes under various conditions, the ambient 
noise which may be encountered in the water will 
vary from one place to another. The level of the am- 
bient noise may be contributed to by noise-making 
countermeasures employed by the enemy. In order 
to protect the receiving system against such noise, 
the level of received signal may be used as a means of 
controlling the receiver sensitivity. In order to pre- 
vent the receiver sensitivity from being controlled 
by the level of the echoes, a large enough time con- 
stant must be used in the control network so that it 
will not respond during the interval of an echo. This 
type of control of receiver sensitivity is known as 
automatic volume control [A VC]. Most echo-ranging 
receiver systems employ both A VC and TVG, al- 
though, the manner in which they are actually em- 
ployed varies considerably from one system to an- 
other. The time constant which is employed in the 
A VC loop will vary with the length of the trans- 
mitted pulse which is used. 

An ordinary listening torpedo responds to the 
noise emitted by the propellers on the target, unless 
some sort of noise-making countermeasure is being 
used. An echo-ranging torpedo responds to the echo 
which is returned by the target; but, when the target 
is moving rapidly through the water, a wake is gen- 
erated, consisting of large numbers of air bubbles 
which extends for a considerable distance aft of the 
target. The wake is an effective reflector, and the 
problem of devising a system which is capable of 
distinguishing between the wake and the true target 
is a difficult one. 

One of the important advantages of an echo-rang- 
ing control system is the fact that one can increase 
the power of the transmitter to quite large values, 
and in this way control the level of echo returned by 
a given target at a given range. If the power of the 
transmitter is increased, the voltage required in the 
transmitter power supply is also increased. The de- 
velopment of suitable power supplies for use in the 
confined spaces of a torpedo presents some prob- 
lems. a 

a See references 1-8 and 41 for additional material on topics 
in this chapter. 


Chapter 16 

MAJOR COMPONENTS 


T he following major components are used in all 
echo-ranging torpedo control systems. 

16.1 TIME BASE 

It is necessary in all echo-ranging systems to use 
a time base to determine the time interval between 
transmitted pulses and the length of the transmitted 
pulses. In addition, it is also necessary to make the 
receiver inoperative during the interval of transmis- 
sion and, in cases where time-varied gain [TVG] is 
employed, to control the time of application of the 
TVG control voltage. The simplest form of time base 
is a system of cam-operated switches to control the 
various events. The cams may be operated by a sim- 
ple motor drive or they may be operated directly by 
the torpedo propulsion motor. In some systems a 
purely electronic timing system is used, employing 
combinations of multivibrator circuits and relays. 
A system using an electronic time base has the ad- 
vantage that the elements of the time base can be 
mounted directly on the chassis containing the rest 
of the electronic gear, whereas the systems using cam- 
operated switches have the advantage of simplicity 
and greater reliability. 

16.2 SIGNAL GENERATOR 

The simplest type of signal generator which has 
been employed is the one used in the German Geier 
system, which consists of a condenser charged to a 
high potential and then discharged by means of a 
cam-operated switch through the tuned circuit of the 
transducer. The type of signal generator more com- 
monly used employs either a power oscillator, which 
can be keyed by means of the time base, or an oscilla- 
tor driving a power amplifier arranged so that both 
the oscillator and power amplifier are keyed by the 
time base. Since the signal generator is operating only 
a small fraction of the total elapsed time, the amount 
of power which can be generated during the actual 
transmitted pulse can greatly exceed the ratings of 
the components for continuous duty. The amount 
which these ratings can exceed the normal continu- 
ous duty ratings depends upon the length of pulse 
employed; for example, in the Ordnance Research 


Laboratory [ORL] project 4 system the transmitter 
is capable of generating about 1.5 kw of electric 
power to be transmitted in 30-msec pulses, at 1.5-sec 
intervals. The power amplifier uses two 829B tubes 
with a plate voltage of 1,500 v. The Bell Telephone 
Laboratories [BTL] 157C system generates 1.5 kw 
of electric power in 3-msec pulses with one pulse per 
second, using a single 829B tube with a plate voltage 
of 3,000 v. It is impossible to use a plate voltage 
higher than 1,500 v with pulses as long as 30 msec 
with the 829B tubes, because the tendency for tube 
breakdown with excessive plate voltage is a function 
of the length of pulse. The power limitation in both 
of these systems is determined by the value of plate 
supply voltage at which breakdown begins to occur in 
the tubes rather than by the power-handling capabil- 
ities of the electrodes. 

16.3 TRANSDUCER 

An important function in an echo-ranging system 
is the conversion of the electric power generated in 
the signal generator into acoustic power in the water 
during transmission, and then the conversion of the 
acoustic power in the returned echo into an electric 
signal which can be applied to the receiver. To do 
this, a single transducer may be used or separate 
units may be used as projectors and hydrophones. 

When the same transducer is used for both pro- 
jector and hydrophone, it is necessary to provide 
some means of protecting the receiver during trans- 
mission. This may be done by use of a switching sys- 
tem, combinations of varistors, or by special design 
of the coupling circuits to prevent excessive voltages 
being applied to the receiver. 

16.4 RECEIVER 

The function of the receiver is to take the electric 
signal generated in the receiving hydrophone, amplify 
the signal, and use the information contained in the 
signal for the purpose of control of the torpedo. In all 
cases, the level of the receiver signal may vary from 
quite low values to quite high values depending on 
the range of the target. It is essential that the re- 
ceiver be capable of handling the expected range of 


70 


RELAY CONTROL SYSTEM 


71 


signal level. This is usually provided for by the use 
of TVG and A VC. The receiver may be used to com- 
pare levels of signal on two separate hydrophones or 
the target bearing may be determined by the phase 
relation between the electric signals generated in the 
two halves of the receiving hydrophone. In the sim- 
plest case, the receiver may be made similar to those 
used in listening-type control systems; however, in 
order to best utilize the advantages of an echo-rang- 
ing system, the receivers usually contain means of 
processing the signal so that real echoes can be dis- 
tinguished from other noises which may be present 
in the water. Since the transmitted pulse is a pure- 
tone signal, the range of frequency of signal to which 
the receiver must be sensitive is normally less than 
in the case of listening-type devices. It is, however, 
necessary to have sufficient range of frequency re- 
sponse to take care of the maximum amount of dop- 
pler shift in frequency which may be present in an 
echo. In some cases, the receiver contains two sep- 
arate systems, one, a steering receiver, which inter- 
prets the signal to determine the direction of control 
necessary for the torpedo, the other, an enabling re- 


ceiver, which determines, on the basis of the charac- 
teristics of the received signal, whether the steering 
receiver should pass its information on to the control 
system. 

16.5 RELAY CONTROL SYSTEM 

In order that the information which comes to the 
receiver be used for actual steering of a torpedo, it 
is necessary for this information to be passed on to 
the engines which control turning of the rudders. 
This is done by some sort of relay control system. In 
some cases only relatively rugged relays are used and 
sufficient amplification is used in order to operate 
these relays directly. In other systems, delicate relays 
are used to operate the rugged control relays and cor- 
respondingly less amplification is used in the elec- 
tronic gear. In addition to the relays which are used 
for actual steering control, relays are also used to de- 
termine when the steering system should be locked off 
the normal gyro control; and in some cases where a 
special enabling receiver is used, the enabling of the 
steering receiver may be done by means of a relay. 


E( 'RET 


Chapter 17 

NATURE OF THE CONTROL PROBLEM 


E cho-ranging torpedoes can be divided into 
two main classes: Those used in antisubmarine 
service and those used in anti surface-ship service. 
The torpedo used in the antisubmarine service is nor- 
mally smaller than that used in anti surface-ship serv- 
ice, because the amount of explosive which is neces- 
sary to disable a submarine is less than that necessary 
to disable a surface ship. Most of the antisubmarine 
torpedoes are launched from aircraft and the use of 
a torpedo of about 7-ft length makes possible the use 
of standard bombing planes for launching. 

The attacks made against submarines using acous- 
tic torpedoes are usually made when the submarine 
is submerged. When the torpedo is launched from an 
aircraft, the location of the submarine is actually un- 
known both in depth and in position. It is therefore 
necessary, as was stated in Chapter 15, to have a tor- 
pedo which is capable of searching both in the azi- 
muth and vertical planes. It is also necessary to have 
a torpedo which can operate at sufficient depth to 
attack a submarine at any feasible operating depth 
for the submarine. 

When the torpedo is launched from an aircraft, it 
is normally launched at the point at which the sub- 
marine was seen to submerge. The torpedo is then 
expected to search until it makes acoustic contact 
with the submarine and then home on the submarine. 
If the torpedo is launched from a surface ship, the 
submarine will normally be located by means of the 
sound gear on board the ship so that the approximate 
bearing and range of the submarine are known at the 
time of launching. When the torpedo is launched 
from a surface ship, it is necessary that some means 
be provided in the torpedo to prevent it from homing 
on the launching ship. This is usually accomplished 
by means of a suitable ceiling switch and a means of 
providing for a certain search interval before acoustic 
homing can begin. 

The method of azimuth search which is used in all 
echo-ranging antisubmarine torpedoes is to have the 
torpedo circle until it makes acoustic contact with 
the submarine. The depth behavior during search 
varies in the different systems. Some are arranged so 
that the torpedo dives fairly rapidly to some fixed 


operating depth where it circles under hydrostatic 
control, whereas in others the torpedo dives slowly 
so that its search is a gradually descending helix. 
This dive may continue until acoustic contact is 
made or until the torpedo reaches bottom, or it may 
be interrupted at some predetermined depth and the 
torpedo level off. 

In the actual acoustic control of an antisubmarine 
torpedo, it is necessary to control the device acous- 
tically both in depth and azimuth. The method of 
acoustic control in the two planes may be the same. 
This is true in the system developed by the Harvard 
Underwater Sound Laboratory [HUSL] where the 
azimuth steering information is obtained by compari- 
son of the electrical phase relation between the signal 
generated in the right and left halves of the trans- 
ducer, and the vertical steering information is ob- 
tained by comparing the electrical phase relation be- 
tween the signals generated in the top and bottom 
halves of the transducer. Somewhat the same pro- 
cedure is used in a version of the British “Dealer” 
device, except that a switch is used in the time base 
which alternates the system from vertical informa- 
tion to azimuth information. In this way a single 
two-channel amplifier is used which alternately op- 
erates for azimuth and depth. The control for both 
azimuth and depth is accomplished by comparison 
of the phase of the electric signal generated in the 
two halves of the transducer. In the system which 
was developed by the General Electric Company, the 
method of securing acoustic-steering information in 
the two planes is different. In their system, compari- 
son of the electrical phase relation between the top 
and bottom halves of the transducer is used for ver- 
tical steering control but the azimuth steering control 
is an on-off control. When echoes are received, the 
azimuth rudders are turned hard to starboard, 
whereas when no echoes are received, the rudders 
turn hard to port. By use of this arrangement, a 
single two-channel amplifier can be used for com- 
parison of the phase of the signals for vertical control 
and the azimuth control is determined simply by 
whether the amplifier is receiving information or not. 

In anti surface-ship service the torpedo may be 


72 


NATURE OF THE CONTROL PROBLEM 


73 


launched by aircraft, surface ships, or submarines. 
The problem of launching an acoustically controlled 
torpedo from a surface ship requires a definite pre- 
caution to prevent the torpedo from homing on the 
launching ship. In the anti surface-ship service the 
bearing and range -of the target are fairly accurately 
known, and the range at which the torpedo is 
launched is normally quite large. All of the devices 
used in this service operate initially under gyro con- 
trol. Provision is made for the torpedo, operating 
under gyro control, to get within acoustic range of 
the target and when acoustic signals are received, the 
acoustic control takes over. In the system developed 
by the Bell Telephone Laboratories [BTL], the 
acoustic control system acts by correcting the gyro 
setting so that the torpedo remains under gyro con- 
trol during the whole period of an acoustic attack, 
the gyro setting being corrected on each received 
echo. The transducer used in this system has a fairly 
wide beam pattern in the azimuth plane so that it 
is able to receive echoes from any target within an 
angle of ± 30 degrees of the torpedo axis. The correc- 
tion on the gyro setting is made by comparing the 
electrical phase relation between the signals gener- 
ated in the two halves of the transducer when the 
echoes are received. In the German Geier system two 
sets of transducers are used which point respectively 
about 40 degrees to port and starboard of the torpedo 
axis. The system does not receive echoes from a point 
directly ahead of the torpedo and the torpedo re- 
mains on a straight gyro course until echoes from a 
target sufficiently off the axis to port or starboard 
are received. When an echo is received on either hy- 
drophone, the steering system is locked off gyro con- 
trol and the rudder is put hard to the side from which 
the echo is received. This condition is held until no 
further echoes are received, when the torpedo drops 
back on gyro control again. With this arrangement, 
if the original firing of the torpedo is sufficiently ac- 
curate to make a hit without acoustic control, the 
acoustic control system will not function. It simply 
functions to correct inaccuracies in the original gyro 
setting. 

In the Ordnance Research Laboratory [ORL] 
project 4 system, a single very narrow beam-pattern 
transducer is used with its axis on the axis of the tor- 
pedo. In order to locate a target off the gyro course, 
the gyro is equipped with a special cam plate which 
causes the gyro course to be “snaky.” The snaky 
course is such that the transducer is able to search 
over an angle of about + 40 degrees. The use of the 


snaky course reduces the forward rate of progress of 
the torpedo by about 5 per cent but it permits the 
use of a single transducer with a very sharp beam 
pattern, thus reducing the self-noise problem to a 
minimum. 

In anti surface-ship service, it is not necessary to 
have acoustic control for the torpedo in both depth 
and azimuth. Most of the systems use only azimuth 
control, operating the torpedo at a running depth 
under hydrostatic control such that the torpedo will 
strike the target. Although the use of a vertical steer- 
ing control makes the acoustic system more compli- 
cated, there is an advantage in operating the torpedo 
during the initial portion of the attack at a depth of 
approximately 50 ft, because propeller cavitation 
noise is reduced. This is especially important in the 
higher-speed torpedoes. It is conceivable that the 
vertical control might be accomplished by the use of 
two hydrostatic-controlled running depths, one used 
during search at the beginning of the acoustic attack 
and the other to be assumed after the attack has 
progressed for a certain length of time. However, 
this arrangement presents real difficulties, and the 
use of the acoustical information to control both the 
depth and azimuth steering seems to be the most de- 
sirable. The echo-ranging torpedo presents one defi- 
nite advantage over the passive acoustic torpedoes 
in the operation of the depth-steering control, in that 
the echo-ranging system is itself a range-measuring 
device. One can introduce a range-measuring element 
in the time base which will permit depth steering 
only after the range has been reduced to a certain 
predetermined value. 

In order to prevent the torpedo from oscillating 
through an excessive angle in the vertical portion of 
its attack, the vertical control should be introduced 
by the application of a correction on the normal 
hydrostatic control system. The simplest means of 
doing this is by application of a mechanical bias to 
the pendulum used in the hydrostatic control. 

The normal torpedo steering control in the azi- 
muth plane is one in which the rudder is thrown 
either hard to port or hard to starboard. A torpedo 
which is under gyro control normally requires the 
rudder to be thrown when the torpedo is a fraction 
of a degree off-course, resulting in a nearly straight 
trajectory for the torpedo. Under echo-ranging acous- 
tic control, the angle off-course required to produce 
a rudder throw is greater than that necessary under 
gyro control and, in addition, an echo-ranging con- 
trol system receives information intermittently. The 


74 


NATURE OF THE CONTROL PROBLEM 


time interval between successive echoes is a function 
of the acoustic range of the torpedo and may be con- 
siderably greater than 1 sec for long ranges. In the 
design of an echo-ranging system, it is necessary to 
take into account the acoustic range which is to be 
used and, therefore, the interval between transmitted 
pulses as well as the dynamic behavior of the torpedo 
body. The way these factors affect the reliability of 
the system will depend also on the sharpness of the 
beam pattern of the transducer and the actual method 
of acoustic control. 

In the General Electric NO 181 and in the German 
Geier devices, the azimuth steering is “on-off.” Al- 
though there is actually considerable difference be- 
tween these two arrangements, essentially the de- 
vices steer one way when echoes are not received and 



0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.4 2.6 


TIME ELAPSED FROM MOMENT OF RUDDER OPERATION IN SECONDS 

Figure 1 . Orientation of body as a function of time 
following beginning of rudder throw. 

che other way when echoes are received. It is neces- 
sary, with this arrangement, that the rate of turn of 
the torpedo be such that it is impossible for it to 
turn completely across the beam between echoes. 
For a given rate of turn of the torpedo body, the 
width of beam pattern required will be a function of 
the maximum acoustic range of the torpedo. 

In systems like the HUSL NO 181 and the BTL 
157C, the steering is controlled by the electrical 
phase relation between the signals generated in the 
two halves of the receiving transducer. This means 
that it is necessary for an echo to be received in order 
to steer the torpedo in either direction. The trans- 
ducer used in the BTL system has a very wide beam 
pattern in azimuth because the torpedo searches on a 
straight gyro course and utilizes the width of the 
beam pattern as its means of making initial acoustic 


contact with the target. As soon as acoustic contact 
is made, the signal is used to correct the gyro setting. 
The dynamics of this body will therefore introduce 
no problem in maintaining acoustic contact with the 
target once acoustic contact is made. In the HUSL 
NO 181 system, however, a transducer with very 
sharp beam pattern is used. The initial search in this 
device is circling and, as soon as acoustic contact is 
made, the rudders are held in either the port or star- 
board position depending on the phase relation of the 
signal generated in the transducer by the last re- 
ceived echo. The dynamics of the torpedo body are 
quite important in this device. The ping interval 
used is 0.65 sec, the rate of turn of the body is 12 de- 
grees per sec, and an interval of 1 sec is required for 
the rudders to turn from one extreme limit to the 
other because the steering is done by means of an 
electric motor. The effect of the dynamics of this 
body on the relative sensitivity of the acoustic sys- 
tem can be determined by considering the solution 
of equation (1) 


d 2 0 dd 

Q~T7 2 + R-r = L(t). 
dt 2 dt 


( 1 ) 


This is the equation of motion of such a body under 
the influence of the torque introduced by the rudder. 
Q is the moment inertia of the body. R is the angular 


Table 1 


Echo no. 

Angle 

(degrees) 

Rel. sig. 
130 yd 
(db) 

Rel. sig. 
260 yd 
(db) 

Rel. sig. 

520 yd 
(db) 

1 

- 8.8 

- 9.1* 

- 7.5* 

- 5.6* 

2 

-15.1 

-30.9 

-29.3 

-23.3 

3 

-14.3 

-34.0 

-35.0 

-33.7 

4 

- 8.8 

-13.5 

-15.9 

-21.6 

5 

- 1.5 

- 1.0 

- 2.2 

- 5.6 

6 

+ 6.3 

- 4.6* 

- 3.5* 

- 3.2* 

7 

+ 12.6 

-23.0 

-19.3 

-15.0 

8 

+ 11.8 

-22.4 

-23.4 

-22.2 

9 

+ 6.3 

- 7.6 

- 9.4 

-13.5 

10 

- 0.9 

0.0 

- 0.5 

- 3.1 


* Steering must take place. 


damping resistance, L(t) is the rudder torque as a 
function of time and 6 is the spacial orientation of 
the body axis. Q/R is called the relaxation time of 
the body, which is the time, measured from the time 
of rudder throw, necessary for the body to come back 
to the same spacial orientation which it had at the 
time of rudder throw. L/R is the rate of turn of the 
body measured in degrees per second. The HUSL 


NATURE OF THE CONTROL PROBLEM 


75 


NO 181 body had a value of relaxation time of 0.5 
sec and the rate of turn was 12 degrees per sec. 


Table 2 


Echo no. 

Angle 

(degrees) 

Rel. sig. 
130 yd 
(db) 

Rel. sig. 
260 yd 
(db) 

Rel. sig. 

520 yd 
(db) 

1 

-3.6 

- 1.0* 

- 1.0* 

- 2.0* 

2 

-9.9 

-13.8 

-11.2 

- 8.3 

3 

-9.1 

-13.4 

-14.1 

- 13.3 

4 

-3.6 

- 3.0 

- 4.3 

- 7.0 

5 

+3.6 

- 1.0* 

- 1.0 

- 2.0 

6 

+9.9 

-13.8 

-11.2 

- 8.3 

7 

+9.1 

-13.4 

-14.1 

-13.3 

8 

+3.6 

- 3.0 

- 4.3 

- 7.0 


* Steering must take place. 


The curve in Figure 1 shows the orientation of the 
body as a function of time following the beginning of 
the rudder throw. The data below show the relative 
level of the received signal at 130 yd, 260 yd, and 520 
yd for the most favorable and the most unfavorable 


phase relation between body orientation and trans- 
mission of the pulses. Table 1 is for the most un- 
favorable condition and Table 2 is for the most fa- 
vorable condition. 

It is obvious that in order to receive every echo, 
the echo-to-reverberation ratio would have to be ex- 
tremely high. Actually it is not necessary to receive 
every echo since the steering system holds in the 
position of the last steering indication. It is therefore 
necessary to have only about 11 db echo-to-rever- 
beration ratio to steer in the worst case considered in 
Table 1 and 8 db ratio to steer in the worst case in 
Table 2. 

When the steering in the vertical plane is accom- 
plished by the same type of hunt as is used in the 
azimuth plane, the condition for combined steering 
in both planes can be considerably worse than that 
for one plane. The use of glide-angle control in the 
vertical plane makes the effect of the dynamics of 
the body on acoustic performance much less serious. 




Chapter 18 

GENERAL ELECTRIC N0181 SYSTEM 


18.1 INTRODUCTION 

I n 1942 a research group of the General Electric 
Company started development of an echo-ranging 
control system to be used in an antisubmarine tor- 
pedo. The system was designed as a conversion sys- 
tem for a torpedo which was already in use as an 
antisubmarine listening torpedo. In August 1944, 
their experimental units were tested and the device 
was accepted by the Navy for production. The pro- 
duction engineering was undertaken by the Leeds 
and Northrup Company, and their first preproduc- 
tion units were tested during the summer of 1945. 

LAG 
LINE 

r~i 


no provision for automatic volume control [A VC]. 
The device is protected against steering on noise 
peaks by the use of an amplitude gate with a time 
constant which requires a substantial part of the 
30-msec pulse to enable. The pulse interval of 0.7 
sec makes the maximum theoretical range about 560 
yd, but performance tests on the preproduction 
models have all been made at 330 yd. The device is 
intended to be dropped from an aircraft at the point 
where a submarine has submerged. It searches in a 
circle with the depth gradually increasing until acous- 
tic contact is made. It then homes on the target. 


FIRST 

AMPLIFIER 


mr 


TVG 


SECOND 

AMPLIFIER 


THIRD 

AMPLIFIER 


TWIN 

RECTIFIER 


COMPARISON 

BRIDGE 


ECHO 

i-l a 

RUDDER 
CIRCUITS 


RUDDER 

STEERING 

MOTOR 


ELEVATOR 

CIRCUIT 


PENDULUM 

CIRCUIT 


ELEVATOR 

STEERING 

MOTOR 








POWER 



TVG 


AMPLIFIER 

OSCILLATOR 


RECTIFIER 


Figure 1. Block diagram of General Electric N0181. 


FIRST 

AMPLIFIER 


SECOND 


THIRD 

AMPLIFIER 


AMPLIFIER 


Figure 1 shows a block diagram of the system. It 
consists of a transmitter capable of generating about 
100 watts of electric power to be transmitted in 30- 
msec pulses at 0.7-sec intervals. A single transducer 
is used as both projector and hydrophone. The re- 
ceiver contains two types of steering-control systems, 
one for the azimuth control and the other for depth 
control. The operating frequency is 60 kc. The re- 
ceiver is under time-varied gain [TVG] control with 


18.2 TRANSDUCER 

The transducer used in this device was developed 
at the Harvard Underwater Sound Laboratory 
[HUSL] and consists of an array of magnetostrictive 
elements. Figure 2 shows the general construction of 
the transducer and the individual elements, while 
Figure 3 shows the arrangement of the elements in 
the array. The numbers indicated on each element 


76 



TRANSDUCER 


77 


CYCLE" WELD 




Figure 2. Details of transducer construction. 


ACTIVE FACE 


WINDINGS CUSHIONED 
WITH AIR CELL NEOPRENE 


100 LAMINATIONS OF 
.005 ANNEALED NICKEL 


.239 


.479 


.479 


.239 


.346 


.692 


.692 



.442 


.884 


.884 


.442 


.500 



.500 


.500 


.442 


.884 


.884 



.346 


.692 


.692 


.346 


.239 


.479 


.479 



Figure 3. Array of elements in the transducer. 

in this figure indicate the relative number of turns 
in the windings. The variation in the windings on the 


units is for the purpose of controlling the pattern of 
the transducer. Figure 4 shows the frequency re- 
sponse curve of one of the transducers produced by 
the Leeds and Northrup Company, Figure 5 shows 
the horizontal pattern, and Figure 6 shows the ver- 
tical pattern. The following are the numerical char- 
acteristics of a representative transducer: In the 
vertical plane, the pattern is 10 db down at 10 de- 
grees off the axis on either side, while in the horizon- 
tal plane the pattern is 7 db down at 10 degrees off 


CD 

Q 


O 

> 


oc 

o 


z 

§ 

o 



30 40 50 60 70 80 90 100 NO 120 130 

KILOCYCLES PER SECOND 

Figure 4. Tranducer frequency response. 


the axis and the overall directivity index is — 22.5 
db. In the process of adapting the transducer to pro- 
duction, the impedance per unit was changed some- 
what so that the impedance per half section comes 
out 90 + ,;130 ohms. The receiving sensitivity is 82 
db below 1 v per dyne per sq cm and, during trans- 
mission, 66 db above 1 v per dyne per sq cm is de- 
veloped at a range of 1 meter. The efficiency in trans- 
mission is about 35 per cent and, it is believed by the 
Leeds and Northrup people that the latest units 
which they have produced are somewhat better than 
this. 


78 


GENERAL ELECTRIC N018I SYSTEM 


18.3 


TIME BASE 


means of a switch which is operated by one of the 
The time base of the General Electric system is a cams. A second switch is used to blank the receiver 
simple cam-operated set of switches. The cams which during the time of transmission by applying - 26 v 


330 ° 


300 


270 


240 ° 



210 180 150 ° 

Figure 5. Transducer horizontal directivity pattern. 


330 ° 


60 ° 300 ° 


270 ° 


240 ° 



120 ° 


210 ° 180 ° 150 ° 

Figure 6. Transducer vertical directivity pattern. 



operate the switches are on a shaft which is driven directly to the screens of the tubes in the receiver, 
by the main motor shaft. The transmitter is operated Three other cam-operated switches are used to con- 
by keying the plate supply of the signal generator by trol the range of the torpedo. The first is a single- 


TIME BASE 


79 



Figure 8. Lag line and TVG balance control. 


50/I/if 



BLANK 

Figure 9. R-F amplifier and rectifier. 


pole double-throw switch which is closed in one posi- 
tion during the time that echoes can be received from 
ranges less than 500 ft. It is closed in the other posi- 
tion during the time that echoes can be received from 
ranges greater than 500 ft. This is used to limit the 
range to 500 ft after echoes have once been received 
within this range. The second is closed by a cam dur- 
ing the time that echoes can be received from ranges 
between 1,000 and 2,000 ft, and the third is closed 


during the time that echoes can be received from 
ranges between 1,500 and 2,000 ft. By means of these 
last two switches, it is possible to decrease the range 
of the torpedo to 1,000 ft, if the body is tilted so that 
it points toward the bottom at an angle greater than 
9 degrees and to 1,500 ft, if the body is tilted so that 
it points toward the bottom at an angle greater than 
6 degrees. 


80 


GENERAL ELECTRIC NQ181 SYSTEM 


18.4 TRANSMITTER 

The schematic of the transmitter is shown in Fig- 
ure 7. It consists of two 6L6 tubes used as power 
amplifiers with their grids driven in parallel by a 
single 60-kc oscillator. The output of one of the 
power amplifiers is supplied to the top half of the 
transducer while the output of the other power am- 
plifier is supplied to the bottom half of the trans- 
ducer. The amplifiers are so connected to the trans- 
ducer windings that the acoustic signal emitted from 
the whole face of the transducer is in phase. The 


at the two grids G1 and G2 will be in phase. If the 
acoustic signal comes from a point above or below 
the axis of the torpedo, the phase of the signals on 
the two halves of the transducer will be different and 
therefore the signal at G1 will no longer be in phase 
with the signal at G2. Because of the presence of the 
lag line in the plate circuits of VI 04, the relative 
values of the voltages at A and B will depend on the 
phase relation of the signals at Gl and G2. The sig- 
nals at A and B are applied to the grids of the tubes 
V105 and V106. These are the first stages of the two- 



transmitter is keyed by means of a switch, operated 
by the time base, which keys the plate and screen 
supply of the power amplifiers. A sample of the 60-kc 
signal generated on the screens of the power ampli- 
fier is rectified by means of the TVG diode and stored 
on a condenser to serve as the gain control bias dur- 
ing the listening interval. 

18.5 RECEIVER 

When an acoustic signal is received by the trans- 
ducer the voltages developed on the two halves are 
impressed on the grids Gl and G2 of V104, Figures 
7 and 8. If the acoustic signal arrives on the trans- 
ducer face from a point dead ahead, the voltages de- 
veloped in the windings of the two halves of the 
transducer will be in phase and therefore the voltages 


channel amplifier for the receiver. The double poten- 
tiometer is used to balance the inputs to the two 
channels and adjust the relative values of TVG vol- 
tage on the grids of V105 and V106. Figure 8 shows 
the details of the circuit from the grids Gl and G2 
to the input of the two-channel amplifier. 

After transmission of a pulse by the system, a con- 
tinuous signal will be received by the transducer be- 
cause of reverberation in the water. The level of this 
signal decays quite rapidly with time. The time re- 
quired for an echo to be received after transmission 
is directly proportional to the range of the target; 
therefore, the level of the echo can be expected to de- 
crease as the time interval between transmission and 
reception increases. A TVG system is used in order 
to control the sensitivity of the receiver so that it 


COMPARISON BRIDGE 


8L 


will not respond to the reverberation but will increase 
in sensitivity with time following transmission, so 
that weak signals from long ranges can be used for 
control. This variable gain is obtained by means of a 
variable bias applied to the grids of V105 and V106. 
The bias is obtained by rectifying a portion of the 
signal appearing on the screen of one of the trans- 
mitter power-amplifier tubes by means of the TVG 
diode, Figure 7, and charging the TVG condenser, 
Figure 8, with the output of this diode. Immediately 
after transmission stops, the condenser starts to dis- 
charge through the 0.24-megohm resistor to ground 
thus producing the time variation in bias. No A VC 
is used in this device. 



The two-channel amplifier consists of two stages 
of tuned amplifier for each channel with a system of 
balancing potentiometers between the two stages to 
be used to equalize the gain in the two channels. The 
output of each channel goes to a diode rectifier. These 
rectifiers are so connected to a bridge that an echo 
pulse is obtained at one point on the bridge network 
whenever an echo is received by the transducer. The 
difference in polarity between two other points on 
the bridge indicates whether the echo came from 
above or below the axis of the transducer. The two- 
channel amplifier is shown in Figure 9 and the com- 
parison bridge is shown in Figure 10. 

18.6 COMPARISON BRIDGE 

There are two conditions to be considered in the 
comparison bridge shown in Figure 10. Condition 1, 
when the target is in the same horizontal plane as 
the axis of the torpedo. In this case, the voltages 
Ei and E 2 will be equal and 

Er = Ei = E 2 , 


and 


E l 


Ei — Ei 


= 0 . 


Condition 2, when the target is not in the same hori- 
zontal plane as the axis of the torpedo. In this case, 


Er 


Ei E 2 

) 


and 


E l = 


E 2 - Ei 


2 



STEERING 

MOTOR 





PENDULUM 


— o 


PEND POS 
MOTOR 


SEARCH 
ANGLE SW 


SEARCH 





SLC 

RELAY 


PEND POS 
MOTOR 


PURSUIT 

Figure 12. Schematic of depth-steering conditions. 


Since the input voltages A and B of Figure 2 are not 
the same for condition 2, the values of Ei and E 2 
will be different. Er will always be negative regard- 
less of the orientation of the target relative to the 
axis of the torpedo. E L will be positive when the 
target is on one side of the torpedo axis and negative 
when it is on the other side. 


2 


82 


GENERAL ELECTRIC NQ181 SYSTEM 


18.7 CONTROL SYSTEM 

This control system utilizes two independent steer- 
ing systems, one for azimuth control and the other 
for depth control. In the search condition the azi- 
muth control system maintains the rudders in the 
port position so that the torpedo searches in a 
port circle. When an echo is received, the voltage E R 
developed in the comparison bridge causes the echo 
trip-relay to open, which in turn causes the rudders 
to swing to starboard. The torpedo then goes into a 
starboard circle. The holding time of this circuit is 


condition, and third, the pursuit condition. The ele- 
vators are controlled during all three of these condi- 
tions by means of a set of contacts operated by a 
pendulum. The frame in which the pendulum con- 
tacts are mounted can be rotated by means of a small 
motor and the various steering conditions are con- 
trolled by the positioning of this frame. Figure 11 
shows the relationship of the pendulum frame, the 
pendulum, the steering motor, and the elevators, 
whereas Figure 12 shows the electric circuit setups 
for the three steering conditions. During the initial 
dive the steering is controlled by the ceiling switch 


I 


r 


O 


r tf’C7T L 

UP 


DOWN 

— \SlSlSLr- 

VERTICAL STEERING MOTOR 


PENDULUM 

— r 


LIMIT 

SWITCHES 


SEARCH 
ANGLE SWITCH 


WNA 


SLC 

RELAY 


UP 

DOWN 


i_J 


SEARCH 

1 


PURSUIT 


n. 




ECHO TRIP 
RELAY 




■A/W- 


»VHt 


ABOVE 

CEILING 

l 


\ 


■c> 


BELOW 

CEILING 


PEND POS 
MOTOR 




i 


-24V 


-48V 


A/W 




15° DIVE 
LIMIT SWITCH 

Figure 13. Schematic of depth control. 


such that the rudders will be held from one echo to 
the next and will continue to hold for about 1 sec 
after the last echo of a sequence has been received. 
At the end of this holding time the rudders will again 
drop back to the port position. This constitutes an 
on-off steering system for the azimuth control, and 
the torpedo will continue to steer on and off the 
beam of the transducer in the azimuth plane during 
the entire course of an attack. 

The vertical steering system is considerably more 
complicated. In dealing with this system there are 
three steering conditions to be considered. First, the 
condition during the initial dive, second, the search 


which operates at a depth of 50 ft, and by the dive 
angle limit switch which limits the dive angle to 15 
degrees. The ceiling switch opens at 50-ft depth and 
throws the control over to the search angle switch 
which causes the torpedo to dive at a fixed angle of 
3 degrees until a depth of 225 ft is attained, when the 
search angle is changed to 2 degrees. If an echo is re- 
ceived from a target the pursuit relay is opened and 
the pursuit condition is set up. Under this condition 
the positioning of the pendulum frame is controlled 
by the SLC relay. A schematic of all of these controls 
is shown in Figure 13. Figure 14 shows schematically 
the functioning of the various relays in an attack. 


BLANKING CIRCUITS 


83 


18.8 BLANKING CIRCUITS 

The following blanking circuits are used to protect 
the device against steering on false information. 

1. The 125-ft blank. This prevents echoes from 
less than 125 ft range from controlling the torpedo. 


duction is permanent unless the torpedo misses the 
target, in which case the range returns to normal. 

3. The 6- and 9-degree blanks. These are range- 
reduction circuits designed to avoid bottom reflec- 
tions. When the pitch of the torpedo reaches 6 de- 
grees down, the range is reduced to 1,500 ft. Also 


PING NO. I 2 3 4 5 6 7 8 9 10 II 12 


PINGS AND 

ECHOES 

Up 

1 

Up 

1 



1 Up 

1 






1 Up 

± 




1 

Down 

1 

Down 









1 

Down 


Sequence of echoes Stort of new 

sequence of echoes 


ECHO 

RELAY 



L_TT_ 


Each drop out of this relay when unaccompanied by an elevotor "drop out" 
causes the body to odd I degree to pitch. 


ELEVATOR 

RELAY 


OUT 


i_n_j 


Each drop out of this relay causes the body to subtract I degree from pitch. 
This relay cannot trip alone without an accompanying trip of the echo relay. 


RUDDER 

RELAY 


Port search turn 



PURSUIT 

RELAY 



This relay holds out for about 10 pings after the last echo. Since the 
echo sequences are seldom separated by more than seven pings, 
once tripped it generally holds out tor the entire chase. 

Figure 14. Schematic of operation sequences. 


Without this provision the torpedo will go around 
the bow of the target. 

2. The 500-ft blank. This is a range-closing cir- 
cuit. Once an echo has been received from a range 
less than 500 ft the range is reduced to 500 ft. This 
range reduction is used to prevent steering on echoes 
which are the result of reflection from the target to 
the bottom then back to the torpedo. This range re- 


when the pitch reaches 9 degrees the range is reduced 
to 1,000 ft. These circuits are all operated by means 
of cams and microswitches operated from the main 
motor shaft. The torpedo pitch is measured by mer- 
cury switches. 

Figure 15 is a schematic of the complete blanking 
system. The switches Cl, C2, C3, and C4 are oper- 
ated by the cams driven by the main motor. Blank- 


84 


GENERAL ELECTRIC NQ181 SYSTEM 


ing is achieved by applying — 26 v directly to the 
screens of the first stages of the two-channel ampli- 


-26V 



PURSUIT 

RELAY 


Figure 15. Schematic of blanking circuits. 

her. The cam switch Cl is closed during the time 
echoes from ranges 0 to 125 ft can come in. This is 
called the 125-ft blank and it prevents steering on 


echoes from ranges less than 125 ft. The two mer- 
cury switches marked 9 degrees and 6 degrees are 
closed for angles greater than 9 degrees and 6 de- 
grees of pitch respectively. The cam C2 is closed 
after the time necessary for receipt of echoes from 
ranges greater than 1,000 ft and C3 is closed after 
the time necessary for receipt of echoes from 1,500 
ft. With this arrangement, if the pitch of the torpedo 
is greater than 6 degrees the receiver will be blanked 
for any echo from a range greater than 1,500 ft, 
whereas if the pitch is greater than 9 degrees the re- 
ceiver will be blanked for any echo from a range 
greater than 1,000 ft. The cam switch C4 is a single- 
pole double-throw switch which is closed in the bot- 
tom position during the time when echoes could be 
received from ranges less than 500 ft and after this 
time it is closed in the top position. If an echo is re- 
ceived from a range less than 500 ft the echo relay 
will also be closed so relay B will be energized. The 
contacts Bl, B2, and B3 are contacts which are 
closed when relay B is energized. When this relay is 
once closed it will stay closed as long as the pursuit 
relay is closed. This means that via C4 and B3 the 
receiver will be blanked for all echoes of ranges 
greater than 500 ft and this blanking condition will 
remain until the pursuit relay opens. a 

a See references 6, 9, 10, 14, 20, 26 and 28 for additional 
material on topics in this chapter. 


Chapter 19 

HARVARD UNDERWATER SOUND LABORATORY N0181 SYSTEM 


19.1 INTRODUCTION 

T he harvard Underwater Sound Laboratory 
[HUSL] undertook the development of an 
echo-ranging torpedo in the summer of 1942, utiliz- 
ing a doppler-controlled enabling system as a pro- 
tection against steering on surface, bottom, or wake 
echoes. A special transducer was developed to oper- 
ate at 60 kc which could be utilized both as a pro- 
jector and a hydrophone. The torpedo was designed 
as an antisubmarine weapon and the electronic gear 
was so designed that it could replace the standard 
electronic gear in the project 61 mine. After prelim- 
inary field tests on two experimental units, six elec- 
tronic panels were built to convert the project 61 
mine to echo ranging with all components engineered 
so that the system could be put in production with 
a minimum of redesign. This design was designated 
as N0181F. 

The units were tested against an echo repeater 
used to simulate a submarine in August 1944. The 
performance in these tests was quite satisfactory. 
The units successfully attacked an echo repeater 
after bench tests indicated that the electronic gear 
was in satisfactory operating condition. Some diffi- 
culty was encountered in the tests against a sub- 
marine, because of the difference between an echo 
repeater and a submarine as a target. The revisions 
necessary in the electronic gear in order to overcome 
these difficulties would not have been serious. How- 
ever, since the General Electric version of NO 181 
had already been selected for this application it 
seemed desirable to do some major revision to sim- 
plify the system and to increase the transmitter 
power output. This program resulted in the develop- 
ment of the Ordnance Research Laboratory [ORL] 
project 4 version which will be described in Chapter 
21 . 

The system uses a single transducer made up 
of 32 small laminated magnetrostriction elements 
mounted on a rubber diaphragm about 6 in. in diam- 
eter. The.32 elements are divided into four quadrants 
and the elements in each quadrant are connected in 
series. The terminals for each quadrant are then 
brought out separately in a cable. Three transformers 


are provided for coupling the transducer to the elec- 
tronic gear, one to match the plate circuits of the 
transmitter power amplifier to the transducer and 
two which match the transducer to the grids of the 
tubes in the first stage of the receiver. By means of 
these transformers, a series-tuned circuit and eight 
resistors, the following features are achieved: 
(1) Transmission and reception are performed with 
the same transducer elements without any form of 
switching being necessary; (2) the plate circuit of the 
transmitter power amplifier is properly matched to 
the transducer; (3) 20 db of voltage gain is provided 
between the transducer and the input of the re- 
ceiver, and, (4) five signals are provided at the input 
of the receiver. The voltage difference between one 
pair is produced by a phase difference between the 
signals on the two port quadrants and the two star- 
board quadrants of the transducer. Similarly, the 
voltage difference between another pair is produced 
by a phase difference between the signals on the two 
top quadrants and the two bottom quadrants of the 
transducer. It is possible therefore to steer in both 
azimuth and depth with the information thus pro- 
vided. The fifth signal is a measure of the total acous- 
tic signal on the transducer and is used to operate 
the enabling system. 

A block diagram of the electronic gear is shown in 
Figure 1. The operating frequency is 60 kc. The 
transmitter power output is 80 watts of electrical 
power with the transducer efficiency about 35 per 
cent. The transmitted pulses are at 0.65-sec intervals 
and the length of the transmitted pulses is 30 msec. 
Since this is an antisubmarine device, acoustic steer- 
ing control is in both azimuth and depth. No time- 
varied gain [TVG] is used but an automatic volume 
control [AVC] is used to allow the receiver sensi- 
tivity to be controlled by the level of the reverbera- 
tion. The enabling of the system is done by a com- 
bination of an amplitude gate and a doppler gate. 
The amplitude gate requires that an echo have a 
level at least 5 db above reverberation level and that 
it persist for at least 5 msec. The doppler enabling 
system sets a requirement of 60 c frequency differ- 
ence between the reverberation and an echo. The 


85 


86 


HARVARD UNDERWATER SOUND LABORATORY N0181 SYSTEM 


system was designed to convert the project 61 listen- 
ing torpedo to an echo-ranging torpedo. The body 
used is the standard project 61 body with very little 
modification besides the substitution of the echo- 
ranging electronic gear for the listening electronic 
gear formerly used. 


in the horizontal plane. Figure 3 is the pattern ob- 
tained when 0.55 watt of power is transmitted con- 
tinuously. Figure 4 is the pattern obtained when an 
electric power of 1.38 kw is transmitted in 1-msec 
pulses. It will be noted that the pattern at high 
power levels is somewhat sharper than at low power 



NuLUF/tATJON 


Figure 1. Block diagram of Harvard N0181 F. 


19.2 TRANSDUCER 

The transducer used was made up of an array of 
laminated-stack magnetostrictive elements. These 
elements were identical with those used in the Gen- 
eral Electric device described in Chapter 18 and il- 
lustrated in Figure 2 of Chapter 18. The version of 
this transducer used in the HUSL system differs in 
the arrangement of the units in the transducer and 
the diameter of the diaphragm, since in this case a 
symmetrical system in the vertical and azimuth 
planes is desired. Figure 2 shows the arrangement of 
the elements in this transducer and the numbers in- 
dicated on the elements are proportional to the num- 
ber of turns in the windings on the elements. This 
shading of the windings on the elements is for the 
purpose of producing the desired patterns. The fre- 
quency response of this transducer is similar to that 
of the one used in the General Electric version and 
is indicated in Figure 4 of Chapter 18. Figures 3 and 
4 of Chapter 19 show the pattern of the transducer 


levels because the four central elements are approach- 
ing saturation. The curves of Figures 3 and 4 show 
the dynamic sound pressure at 1 m as a function of 
the angle. The following are the numerical charac- 
teristics of a representative transducer. The pattern 
is 9 db down, 10 degrees off the axis. The overall 
directivity index is — 23 db and the first minor lobes 
are at least 25 db down. The impedance of each 
quadrant is 25 + j80, and since the transducer is 
connected with the quadrants in series-parallel, this 
is the impedance of the entire transducer in actual 
use. During transmission, 74 db above 1 dyne per 
sq cm is developed at a range of 1 m per volt of elec- 
tric signal applied to the transducer. The receiving 
sensitivity is —86 db below 1 volt per dyne per sq 
cm. The, efficiency reaches a maximum value of 38.5 
per cent at an electric power input of 800 watts. At a 
power input of 1.38 kw the acoustic power output is 
403 watts with an efficiency of 29.2 per cent. Since 
the transducer is symmetrical, its pattern in the ver- 
tical plane is identical to the pattern in the hori- 
zontal plane. 



INPUT CIRCUIT 


87 


19.3 INPUT CIRCUIT 

The input circuit used has the advantage that it 
combines the step-up transformation from the trans- 
ducer impedance to the grid impedance with a lag- 
line action all in ojne step. Furthermore, it permits 
transmission without need for disconnecting the re- 
ceiving system. The circuit for this input system is 
shown in Figure 5. In this circuit, quadrants 1 and 
3 of the transducer are connected in parallel as are 
also quadrants 2 and 4, but, as indicated in the dia- 
gram, quadrants 2 and 4 are connected so as to give 
voltages 180 degrees out of phase with quadrants 1 
and 3 when the sound is normally incident on the 



.18 

.30 


.30 

.18 







.18 

.40 

.62 

.62 


.40 

.18 






.30 


.62 

1 


1 

.62 


.30 


.30 


.62 


.62 


.30 


•18 




.62 




.40 


.40 

.62 



.18 












.18 


30 


.30 


.18 


Figure 2. Array of elements in the transducer. 


transducer. The voltages developed across the two 
input transformers thus vanish for sounds normally 
incident and are proportional to the difference be- 
tween the voltages on the corresponding pairs of 
quadrants when the sound is not normally incident. 
It is very important that the coupling between the 
two halves of the input transformer be as tight as 
possible. This is particularly important for trans- 
mission since the transmitting signal is fed through 
the center taps of the input transformer and if the 
coupling is not tight, there will be a large voltage 
drop across the two halves; hence the voltage ap- 
pearing at the center tap will not be the voltage ap- 
pearing across the transducer itself. 

The operation of the circuit shown in Figure 5 
may be best described by computing the voltages 


appearing at various points in the circuit, assuming 
that the voltages developed by each of the four 
quadrants of the transducer, and the impedances, 
are as indicated in Figure 5. It is to be noted that 



Figure 3. Transmitting pattern for 0.55 watt. 


the condensers series tune each quadrant of the 
transducer and similarly the condenser Y a series 
tunes the inductive reactance of the coil X 8 . The 
circuit X s Y 8 will be referred to as the 90-degree cir- 



cuit since it is tuned so that there is a 90-degree 
phase shift between the two ends of X 8 . 

The resistance Rl is assumed to match the in- 
ternal resistance of quadrants 1 and 3 or 2 and 4 in 
series so that 


R l = 2N 2 Ri. 


( 1 ) 
























88 


HARVARD UNDERWATER SOUND LABORATORY N0181 SYSTEM 


If this is the case, the voltages developed across 
Rl are 

\N (e\ — ea), 

and (2) 

- |N(e 2 - e A ), 

on the left and right hand sides of the diagram respec- 
tively. In the circuit as used, Rl was omitted and 
the value of Rb was so selected that the network pro- 
vided the load R L . Neglecting the loading effect of 


* 


* 


«b) - 


jjYs 

R s \R\ 


(ci + 62 + ea + 64) ( 5 a) 


eO 


jjYs 

Rs + T^l 


(ci + 62 + 63 + 64) ( 5 b) 



the 90 -degree section consisting of the series-tuned 
circuit X s Y 8 , the voltage appearing at the point E is: 

t(^1 + 02 + C3 + 64). ( 3 ) 

It is essential that the two halves of the output 
transformer be very tightly coupled in order to 
achieve the result ( 3 ). The factor 34 arises from the 
fact that the voltage is that appearing across only 
one-half the secondary of the output transformer. 
The impedance looking back from point E looks like 
all four quadrants in parallel, namely }4Ri- The 
voltage developed at point F after the step-up cir- 
cuit is therefore given by: 

p ei + e 2 + ea + e 4 ) • (4) 

IXs r 4 Lli 

The voltages appearing at points A, B, C, and D are, 
respectively : 


C: iN- — Rl l -(ei - e 2 ) ^ 


R l + 2N 2 Ri 


D: m 


Rl 

Rl + 2 N 2 R, 


(e 2 — e 4 ) 


R s + \R\ 

(ci + e 2 T* ^3 + 64) ( 5 c) 

jjYs 

Rs + t^i 

C Ci + ^2 + C3 +64) (5d) 


For simplicity of notation, let 
NR l 

Rl + 2 N 2 Rl 
Y 


= Qd’ 


(6) 


R s + t R 


1 D “ 


INPUT CIRCUIT 


89 


In terms of this notation the voltages appearing 
at the four steering grids are as follows: 

RT: — \KQ s (e\ + 62 + e% + e 4) + 

tQd{v 1 + e\ — et — ez), (7a) 
LFT: — ijQs(ei 4 €2 + ez + e±) — 

+ 64 — 62 — ez), (7b) 
UP: —\jQs{e\ 4 - e<L + ez + e 4 ) + 

tC?d (6 1 + 62 — 63 — 64), (7c) 
DN: — \jQ s {e\ 4- e<i + ez + e±) — 

tQz>(c 1 + 62 — e3 — e\). (7d) 

The right and left voltages may be written in the 
form: 


RT: —jV 0(61 + e^)e + ja + + ez)e — ja, (8a) 

LFT: —jV o(ei + e\)e — ja + {e<i + ez)e + ja, (8b) 
when Vo = iQ 8 2 + Qd 2 , 
and 

, Qd 

tan a = — • 

Qs 

It is thus seen that the final output voltages are sim- 
ilar in angular dependance to what would be ob- 
tained with a lag line of angle 2 a between the right 
and left halves of the transducer. Similarly the effect 
of a lag line between the bottom and top halves is 
obtained at the UP and DN outputs. Referring back 
to equation (6) we see that the equivalent lag-line 
angle is given by: 


2a = 2 tan 


_, Qd 

Qs 


2 tan" 


NR 


Rs + T^l 


R l + 2N 2 Ri Y 8 


As an example, in the Harvard N0181F system, 
Q 8 is 10, N is 15.2, and R L = 2 N 2 R if whence Qd = 
7.6. Hence, the equivalent lag-line angle is 74 de- 
grees. 

The transmitting behavior of the circuits is regu- 
lated by the biased diode connected from the secon- 
dary center taps of the input transformers to ground. 
When the voltage at this point exceeds the bias on 
the diode, the latter shunts the condenser Y 8 and 
spoils the Q of the tuned circuit X 8 Y 8 so that this 
circuit, which has been termed the 90-degree circuit, 
does not load the transmitter appreciably. This fea- 
ture requires for its proper operation that the diode 
have a low-resistance d-c return to ground, which in 
the present case is provided by the series coil X 8 and 
the secondary winding of the output transformer. If 
the input transformers are properly center-tapped, 
the voltages developed across the two halves in 
transmission tend to cancel, with the result that the 


inputs provide negligible loading of the transmitter 
even though the secondaries are terminated. Con- 
siderable unbalance in the input system may be tol- 
erated without appreciable loss of power. 

The bias on the diode is adjusted so that it is never 
exceeded by the signal voltage appearing between F 
and ground during reception. In that case the voltage 
appearing at F, and hence on terminal 1, gives sim- 
ply the unshifted receiving pattern, which may be 
used as desired. In this system it provides the signal 
input for the channel which controls the AVC and 
actuates the doppler gate system. 

Figure 6 shows the voltage differential between 
terminals 3 and 4 or 2 and 5 of Figure 5 as a function 
of electrical phase difference between signals on the 
two halves of the transducer determined experimen- 
tally with a representative unit. 



0 10 20 30 40 30 60 70 

SIGNAL PHASE DIFFERENCE IN DEGREES 


Figure 6. Voltage differential as a function of signal 
phase difference between signals from the two halves 
of the transducer. 

Figure 7 shows a set of curves obtained from one of 
the experimental units. These indicate the perform- 
ance of the transducer and input circuit in both 
transmission and reception. The curves indicating 
performance in reception were obtained by mounting 
a source so that it made various angles with the axis 
of the transducer and determining the relative pulse 
level as a function of angle which was necessary to 
actuate the steering relays in the system in one case, 
and in the other the level necessary to operate the 
enabling system. The signal which operates the en- 
abling system is taken from terminal 1 in the diagram 
of Figure 5. The third curve shows the relative level 
of the transmitted pulse as a function of the angle off 
the axis of the transducer and it was determined by 
measuring the levels of signal received by a hydro- 
phone when the transducer was excited by means of 
the transmitter in the torpedo. 


90 


HARVARD UNDERWATER SOUND LABORATORY NQ181 SYSTEM 





Figure 7. Characteristics of transducer and input circuit. 


rt/lS£ ro Q££RA7£ 



Figure 8. Time-base circuit. 


TRANSMITTER 


91 


19.4 TIME BASE 

The system is designed to transmit 30-msec pulses 
at 0.7-sec intervals. In order to accomplish this a 
multivibrator is used as a time base. The transmitter 
is designed so that it is actuated by a positive pulse 


The multivibrator circuit incorporating the tube 
VI in Figure 8 determines both the 0.7 -sec interval 
and the 30-msec pulse length. The resistor R3 and 
the condenser C3 determine the 0.7-sec interval 
while the resistor R1 and the condenser C2 deter- 
mine the length of the pulse. The 30-msec pulse 


_ 30 MILLISEC . 

50 MILLISEC 

P FVFR RF RATION - 


TRANSMISSION 

m “CVCnDLnM 1 IVJW • 

SAMPLING PERIOD 

RECEIVER 



LISTENING PERIOD " 





Figure 9 

Sequences controlled by the time-base circuit. 


and it is also necessary to blank the receiver by 
means of a negative pulse which is generated simul- 
taneously with the transmitter-actuating positive 
pulse. In addition, it is necessary to operate a relay 
immediately following the end of the transmitted 


which is generated by the system is a negative pulse. 
The positive pulse used to actuate the transmitter is 
obtained by means of the phase-inverter stage V2. 
Figure 9 shows schematically the sequences which 
are controlled by the time base. 


ff/S 



pulse and hold the relay closed for a period of 50 
msec. The operation of this relay circuit is accom- 
plished by differentiating the negative pulse by 
means of the condenser C5 in Figure 8 so that the 
relay operates on the positive pip at the end of this 
pulse. A holding time is incorporated in the relay 
amplifier circuit to enable it to hold the relay closed 
for the necessary 50-msec interval. 


19.5 TRANSMITTER 

Since this device utilizes the doppler frequency 
shift caused by the motion of the target through the 
water to enable the steering amplifier, it is necessary 
that the frequency stability of the transmitter be 
quite good. In order to accomplish this, care was re- 
quired in the design of the transmitter oscillator and 





92 


HARVARD UNDERWATER SOUND LABORATORY N0181 SYSTEM 


in the means of coupling the oscillator to the power 
amplifier. The 1-2-3 section of the tube VI in Figure 
10 is the transmitter-oscillator stage. The frequency- 
determining components are the inductance LI and 
and the condensers Cl and C2. The oscillator is 
coupled to a buffer stage by means of the condenser 
C3. The buffer stage uses the 4-5-6 section of the 
tube VI. The 6V6 tube V2 is the driver stage which 
is used to drive the pair of 815 tubes V3 and V4 
which constitute the power amplifier. The oscillator 
and the driver stage are normally biased to cutoff by 
means of the — 48- v connection. The transmitter is 
actuated by means of a positive pulse applied to the 
grid of the oscillator by way of the resistor R4 and to 
the grid of the driver by way of the resistor RIO. 
These stages are then made operative for the dura- 
tion of the 30-msec positive pulse from the time base 
circuit. The output of the power-amplifier stage is 
fed to terminals 6 and 7 in Figure 5. These are the 
terminals of the primary of the power-output trans- 
former which is an integral part of the input circuit. 
The supply voltage for the transmitter is applied to 
the center tap of the primary winding of the power- 
output transformer. The electric power output of the 
transmitter is about 80 watts resulting in an acoustic 
power output from the transducer of about 28 watts. 
The power supply for the screens and the plates of 
the power amplifier as well as the plate of the driver 
stage is a special battery which generates 470 v for 
the plate supply, and a 200- v tap is taken for the 
screen supply. In order to reduce the size of this bat- 
tery, a 40-Aif electrolytic condenser was connected 
between the 470-v terminal and ground, and a 80-juf 
electrolytic condenser was connected from the 200- v 
terminal to ground. This arrangement allows the 
condensers to supply the very high current drain 
necessary during the interval of transmission and the 
battery charges the condensers again during the lis- 
tening period. The plates of the multivibrator tube 
VI in Figure 8 were also driven from the 200-v tap 
on the transmitter battery. Another battery with a 
135-v tap was used for the receiver power supply. 
This was the source for the + 135 v for the transmit- 
ter oscillator and buffer stages. 

19.6 STEERING RECEIVER 

The steering receiver utilizes the output from ter- 
minals 2, 3, 4, and 5 of the input circuit indicated in 
Figure 5. As was pointed out in Section 19.3, the 
voltage difference between terminals 3 and 4 is de- 


termined by the electrical phase difference between 
the signals on the right and left halves of the trans- 
ducer, whereas the voltage difference between the 
signals appearing on terminals 2 and 5 is determined 
by the electrical phase difference between the signals 
on the top and bottom halves of the transducer. If 
an echo is received from a direction to the right of 
the axis of the transducer the voltage on terminal 3 
will be higher than that at terminal 4, whereas if the 
signal is received from a point to the left of the axis 
of the transducer the voltage at terminal 4 will be 
higher than that at terminal 3. In the same way, sig- 
nals arriving from above the axis of the transducer 
produce a higher voltage at terminal 2 than at ter- 
minal 5, whereas signals arriving from below the axis 
of the transducer produce a higher voltage at terminal 
5 than at terminal 2. If the signal arrives from a point 
on the axis of the transducer, the signals developed 
at all four of these terminals will be the same. 

In the steering receiver, shown in Figure 11, the 
signals from these terminals are applied to the grids 
of the tubes VI, V2, V3, and V4. These tubes are 
supplied with a signal on their screens which is gen- 
erated by means of a local oscillator operating at a 
frequency of approximately 1 kc. This oscillator and 
its associated phase-shifting network generates four 
signals of the same frequency, but the phase rela- 
tions are such that if one is taken as 0 degree, there 
will be one at 90 degrees, one at 180 degrees, and one 
at 270 degrees. The four tubes VI, V2, V3, and V4 
with their 1-kc screen supply constitute a switching 
system in which one tube is active for only about 
one-fourth of each cycle of the local 1-kc oscillator. 
The plates of the four tubes are connected together 
and the primary of a 60-kc band-pass filter serves as 
the common plate load for the four switching tubes. 
The output of the band-pass filter is a 60-kc signal 
so modulated by the 1-kc switching system that if a 
signal is fed to the grid of only one of the tubes, there 
will be output from the band-pass filter during only 
about one-fourth of each cycle of the 1-kc oscillator. 
The output of the 60-kc band-pass filter is fed to a 
two-stage resistance-coupled amplifier which in turn 
is coupled to a third stage with a 60-kc band-pass 
filter in its plate circuit. The two-stage resistance- 
coupled amplifier is blanked during the time of trans- 
mission by application of the negative pulse from the 
time base to the suppressor grids in the tubes. These 
two stages are also subject to A VC control applied 
to the control grids of the tubes. The method of ob- 
taining the A VC will be described in a later section. 


STEERING RECEIVER 


93 


The third stage of the 60-kc amplifier contains the 
enabling feature. This stage is biased to cutoff by 
means of a negative bias applied to the suppressor 
grid and the control grid so that a signal cannot 
pass this stage unless a positive enabling pulse is 
supplied to counteract this bias. The enabling pulse is 
supplied from the doppler-enabling receiver which 
will be described in the next section. Following the 
third stage of 60-kc amplifier is a rectifier which 
serves as a demodulator. The output of the demodu- 
lator contains a filter Cl and R7 which removes the 


larity is determined by whether the signal on termi- 
nal 2 or 5 of the input circuit is larger. Each of these 
phase-sensitive detectors has a maximum output 
voltage which is determined by the level of signal 
from the 1-kc oscillator. For small values of target 
angle the voltage output of the phase-sensitive de- 
tectors is a function of target angle, but if the target 
angle is very much more than 1 degree, the voltage 
level of the phase-sensitive detectors will reach the 
saturation value. The outputs of the two phase- 
sensitive detectors are fed to the steering-relay am- 



Figure 11. Steering receiver. 


residual 60-kc signal. The resulting 1-kc signal which 
also contains considerable 2-kc and 4-kc components 
is fed to the grid of V6 which is a 1-kc amplifier with 
tuned plate load of sufficiently high Q so that the 2-kc 
and 4-kc components in the signal are quite effec- 
tively suppressed. The output of the 1-kc amplifier is 
fed by means of two O.Ol-^uf condensers to two phase- 
sensitive detectors. These phase-sensitive detectors 
receive activating signal from the same 1-kc oscillator 
system which is used for switching the input tubes. 
In this way the phase-sensitive detector marked 
azimuth generates a d-c voltage whose polarity is 
determined by whether the signal on terminal 3 or 4 
of the input circuit is larger. The vertical phase- 
sensitive detector produces a d-c voltage whose po- 


plifiers. These amplifiers will be described in a later 
section. 

The sensitivity of the four switching tubes VI, V2, 
V3, and V4 to the 60-kc signal is controlled by three 
potentiometers, PI, P2, and P3, which control the 
level of direct current which is allowed to flow in the 
resistors Rl, R2, R3, and R4. The voltage drop in 
these resistors controls the grid bias of these tubes 
and therefore their sensitivity. It is necessary to pro- 
vide the three potentiometers in order to balance the 
sensitivity of V2 and V3 for the azimuth channel 
and VI and V4 for the vertical channel and then to 
balance the vertical channel relative to the azimuth 
channel. 

When an echo with doppler frequency shift is re- 





94 


HARVARD UNDERWATER SOUND LABORATORY N0181 SYSTEM 


eeived, a pulse is generated in the enabling receiver 
which applies the enabling positive pulse to the 
grids of the last stage of the 60-kc amplifier. After 
the beginning of this pulse the signal is able to ride 
through the last stage of the 60-kc amplifier and the 
demodulator so that the 1-kc amplifier stage is ac- 
tuated. It is necessary to make a compromise in the 


by the frequency difference between the reverbera- 
tion returned from the water surrounding the tor- 
pedo and the echo. In order to do this, a frequency- 
sensitive receiver was necessary for the enabling and 
it was necessary for this frequency-sensitive receiver 
to incorporate an automatic frequency control sys- 
tem in order to correct for the own doppler produced 

7-KC DISCRIMINATOR 



selection of the Q of the tuned circuit in the 1-kc 
amplifier. A very high Q circuit will give maximum 
rejection of 2-kc and 4-kc components while it in- 
creases the time necessary to build up the 1-kc am- 
plifier to full output. A Q of about 15 for this circuit, 
when it is loaded by the phase-sensitive detectors, 
was selected as the best compromise. 

19.7 DOPPLER-ENABLING RECEIVER 

In the design of the Harvard NO 181 system, pro- 
vision was made for enabling the steering receiver 


by the motion of the torpedo through the water. This 
enabling receiver has incorporated in it an amplitude 
gate so that in addition to the criterion of frequency 
difference there is also a criterion that the level of 
echo relative to reverberation must exceed a certain 
value and persist for a certain minimum length of 
time. 

The circuit is indicated schematically in Figure 
12. VI is a stage of amplification which receives its 
input from terminal 1 of the input circuit indicated 
in Figure 5. This signal is proportional to the average 



DOPPLER-ENABLING RECEIVER 


95 


voltage developed on the four quadrants of the trans- 
ducer and has the frequency of the received acoustic 
signal which is nominally 60 kc. V2 is a frequency- 
converter stage with the signal supplied to its control 
grid from the output of the stage VI. The signal 
which is supplied to its screen is the output of the 
oscillator stage V9 whose frequency is nominally 53 
kc. The plate load of V2 is the primary of a 7-kc band- 
pass filter which has a frequency range of 1.4 kc be- 
tween the two 3-db down points. This band-pass 
filter selects the 7-kc frequency difference from the 
combination of frequencies present in the plate cir- 
cuit of V2. The two-stage amplifier which follows the 
7-kc band-pass filter is the same two-stage amplifier 
as is indicated in Figure 11. An important feature of 
this device is the use of a common amplifier for the 
60-kc steering receiver and the 7-kc frequency- 
sensitive receiver. Both of these receivers are under 
A VC which is applied to this two-stage amplifier; the 
source of the A VC voltage is the signal voltage gen- 
erated in the 7-kc channel. Following the two-stage 
amplifier is a third stage which includes a 7-kc band- 
pass filter identical with the one in the plate circuit 
of the stage V2. A portion of the output of this last 
7-kc band-pass filter is rectified for use in the auto- 
matic volume control which will be described in de- 
tail in Section 19.9. Another portion of the output 
goes to the amplitude gate which passes an echo sig- 
nal if it exceeds the background level of reverberation 
by as much as 5 db and persists for as long as 5 msec. 
The amplitude-gate circuit will be described in detail 
in Section 19.8. 

Since the wave form of the signal emerging from 
the amplitude gate is considerably distorted, this 
signal is fed to a 7-kc tuned amplifier which is fol- 
lowed by the stage V3 which contains a discrimina- 
tor primary in its plate circuit. The stage V3 also 
serves as a limiter stage so that the discriminator 
output voltage will be a function of frequency only. 
The action of the discriminator and the rectifier V4 
produces a d-c voltage at A, whose polarity is de- 
termined by whether the frequency is above or below 
the center frequency of the discriminator and whose 
magnitude is determined by the amount of the fre- 
quency difference from the center frequency. The 
filter consisting of R6, R7, and R8 and the condensers 
C7, C8, C9, and CIO is an RC filter which is designed 
to reduce the effect of peaks of voltage at A pro- 
duced by fluctuations in the reverberation frequency. 
The discriminator filter has a time constant which 
adds to the time constant incorporated in the am- 


plitude gate, since it is impossible for the discrimina- 
tor filter to start to build up until after signal has 
begun to pass the amplitude gate. The time constants 
of the amplitude gate and of the discriminator filter 
are important factors to consider in the operation of 
the entire system, since it is necessary that both of 
these time constants be overcome before any ena- 
bling pulse can be applied to the enabling grids in the 
last stage of the 60-kc steering amplifier in Figure 
11. This enabling pulse must arrive at the enabling 
grids in the steering amplifier in time to allow the 

1- kc amplifier to build up to sufficient voltage to 
operate the phase-sensitive detectors. It is the sum 
of all of these time constants which determines the 
minimum length of transmitted pulse which can be 
used to operate the system. It was these factors 
which originally determined the use of transmitted 
pulses of 30 msec. 

The voltage which is supplied at terminal A is 
used for the automatic frequency control which cor- 
rects the local 53-kc oscillator for the own doppler 
of the torpedo. The relay which is operated by the 
differentiated negative pulse so that it is closed dur- 
ing the 50-msec interval following transmission is 
indicated in Figure 12 as the own-doppler nullifier 
[ODN] relay. This relay connects terminal A to the 

2- nf condenser C15 and the grid 4 of V8. V8 is a re- 
actance tube which serves to place a variable resist- 
ance between the condenser C18 and ground. As the 
potential of the grid 4 of this tube is changed, the 
effective resistance between C18 and ground is 
changed; this changes the effective capacitance of 
C18 in the oscillator tank circuit. The frequency at 
which the transmitter oscillator is operated is so 
chosen that with the normal speed of the torpedo, 
the voltage generated at terminal A by the rever- 
beration signal will be zero when the potential to 
which the condenser C15 is charged is as near zero 
as possible. When the torpedo is started, the ODN 
relay samples the voltage developed at A from the 
reverberation following each ping and the condenser 
C15 is gradually charged until the frequency of the 
53-kc oscillator is adjusted to the point where rever- 
beration signal produces zero voltage on the average 
at A. This requires normally from 5 to 10 pulses 
from the transmitter. The rate of correction of the 
ODN is determined by the discriminator sensitivity 
in volts per cycle, the reactance tube sensitivity in 
cycles per volt, and the resistors R23, R24, and R25, 
and the condenser C15. The product of discrimina- 
tor sensitivity and reactance-tube sensitivity is about 


96 


HARVARD UNDERWATER SOUND LABORATORY N0181 SYSTEM 


lei to the axis of the torpedo. In order to provide for 
this frequency difference for enablement, a separate 
45-v battery in the receiver battery pack was con- 
nected to the terminals of R9 and Rll as indicated 
in Figure 12. The voltage drop across the resistors 
RIO and Rll provide the voltage which must be 
overcome by the direct current generated in the dis- 
criminator by means of the frequency difference. The 
value of the required frequency range, called the 
doppler notch width, can be adjusted by varying the 
value of the resistor R9. 



100 and the time constant determined by adjusting 
R25 was made so that when the frequency input at 
the grid of VI was shifted 200 c from that which 
would produce zero voltage at A, four pings would 
cause the voltage at A to be reduced to half the ex- 
treme value; i.e., half correction for the own doppler 
can be achieved in about four pings. Since the nature 
of the amplitude gate is such that a continuous signal 
like the reverberation which is controlling the A VC 
of the amplifier cannot pass the amplitude gate, it 
is necessary to disable the A VC during the period of 

c/ 


w 

C3 



sampling of the potential at A by the ODN relay. 
This is accomplished by another pair of contacts on 
the ODN relay which perform the function of short- 
ing the AVC during the ODN sampling period. 

Since the shape of the pulse envelope emitted by 
the transmitter produces a certain frequency spread 
in the transmitted frequency and since all reverbera- 
tion is not received from points on the axis of the 
transducer, the reverberation will contain a certain 
range of frequencies. It is necessary to impose the 
condition that the doppler shift in frequency caused 
by the motion of the target be greater than a certain 
value in order to avoid enabling of the system be- 
cause of frequency components which are present in 
the reverberation. The choice of the minimum dop- 
pler frequency shift to be used for enablement was 
about +60 c. This corresponds to a component of 
target speed relative to the water of lj/jg knots paral- 


When the pulses have sufficient doppler frequency 
shift to produce a voltage which will overcome the 
bias on either terminal 3 or 8 of V5, the pulse will 
pass one side or the other of the rectifier and the 
pulse signal will be applied to the grid 1 of V6. If the 
pulse supplied to grid 1 is a negative pulse, a positive 
pulse will be generated at the plate terminal 2 of 
V6 and will be passed through the lower half of the 
diode of V7. If, however, the pulse at grid 1 of V6 is 
a positive pulse, the pulse at plate 2 of V6 will be a 
negative pulse which cannot pass the diode V7 but a 
negative pulse will be applied to grid 4 of V6 which 
will generate a positive pulse at plate 5 which will 
pass the upper pair of electrodes in V7. The tube V6 
therefore plays the role of pulse amplifier and phase 
inverter so that, regardless of the polarity of the 
original doppler pulse, a positive enabling pulse will 
be generated at the output of the rectifier V7. The 


AMPLITUDE GATE 


97 


enabling pulse is then applied directly to the sup- 
pressor grid and the control grid in the enabling 
stage of the steering amplifier shown in Figure 11. 

19.8 AMPLITUDE GATE 

The details of the circuit elements in the ampli- 
tude gate are indicated in Figure 13. VI and V3 are 
the two sections of a single 6H6 tube while V2 and 
V4 are the two sections of a single 6SN7 tube. The 
output of the 7-kc amplifier appears at point PI in 
the diagram and this signal is applied via R6 to grid 
terminal 1 of V2 and via R1 and Cl to the cathode 
terminal 4 of VI. Terminal 4 of VI is connected to 
the point P2 by way of the resistor R2, and the anode, 


as happens when an echo is received, will result in a 
corresponding increase in level of signal at PI. This 
increase in level of signal at PI will cause grid ter- 
minal 4 of V4 to become more positive, which will 
change the relative values of the potential at points 
P2 and P3 so that terminal 3 of the diode section V 1 
will no longer be negative with respect to terminal 4 
of VI. When this condition is achieved, the diode 
section VI becomes conducting. The system is so de- 
signed that the amplitude gate becomes conducting 
when the ratio of echo to reverberation level is about 
6 db with the level of reverberation at terminal 1 of 
Figure 5, — 90 db referring to 1 v [dbv]. The echo- 
to-reverberation ratio necessary to pass the ampli- 
tude gate increases with decrease in reverberation 



terminal 3, of VI is connected to the point P3 by 
way of a resistor (R3). When no signal appears at 
PI, P2 is sufficiently positive with respect to P3 so 
that the diode section VI cannot conduct any signal. 
If the signal level at PI is increased, this signal will 
be amplified by V2 and the output signal from V2 
will be rectified by V3 and the resulting bias applied 
to grid 4 of V4. 

Because of the action of the AVC, the signal level 
at PI will remain nearly constant for a considerable 
variation in the level of the input signal to the am- 
plifier. However, the time constant in the AVC cir- 
cuit is such that a sudden change in signal level, such 


level because the AVC does not provide a perfectly 
flat response in the common amplifier. The adjust- 
ment of the echo-to-reverberation ratio at which con- 
duction takes place is done by adjusting the relative 
values of the two resistors, R16 and R17. It is also 
required that the amplitude gate shall remain non- 
conducting unless the echo signal persists for a period 
of at least 5 msec . This time constant is obtained by the 
resistor R12 and the condenser C5. In order to make 
the amplitude gate conducting during the first 50 
msec following transmission, the AVC circuit is 
grounded by means of a contact on the ODN 
relay. 


98 


HARVARD UNDERWATER SOUND LABORATORY N0181 SYSTEM 


19.9 AVC CIRCUIT 

The 7-kc channel is also used as the source of sig- 
nal for the AVC on the 2 stages of the common 
amplifier. The tube VI in Figure 14 is the AVC rec- 


which form a bleeder circuit from the + 135-v supply, 
a delajr bias of 8.4 v is applied to terminal 4 of the 
rectifier. When the peak value of the signal exceeds 
this bias, the diode conducts and charges the con- 
densers C2 and C3. The two-megohm resistor R5 



Figure 15. Characteristics of the N0181 electronic system. 


tifier. The 3-4 section of this tube rectifies the sam- with the capacity of the condenser C3 serve to make 
pie of signal which is obtained from the output of the time constant of the AVC system large enough 
the 7-kc amplifier via the condenser Cl and the so that an echo signal will not be long enough to con- 
resistor Rl. By means of the resistors R2 and R3 trol the AVC sufficiently to prevent conduction of 


AYC CIRCUIT 


99 



Figure 16. Characteristics of the N0181 electronic system. 


Si 


100 


HARVARD UNDERWATER SOUND LABORATORY N0181 SYSTEM 


the full signal by the amplitude gate. In order to be 
certain that the A VC action will diminish at a rate 
sufficient to cause the amplifier to follow the decay 
in reverberation, the A VC condenser C3 is furnished 
with a discharge path by way of the 8-5 section of 
the diode. By this arrangement, the junction of R5 
and C3 will never be more negative than terminal 3 
of the 3-4 section of the rectifier. The condenser C2 
and the resistor R4 serve to take out the 7-kc ripple 
from the voltage appearing at terminal 3. The A VC 
voltage for the first stage of the common amplifier 
is taken from the junction of the resistor R7 and the 
condenser C5 whereas that for the second stage of 
the common amplifier is taken from the junction of 
R5 and C3. The A VC is such that, for a change of 
signal level at terminal 1 of the input circuit of 20 
db, the change in level at PI in Figure 13 is about 
1 db. The removal of the AVC during the first 50 
msec of the listening period is accomplished by con- 
necting the junction of the resistor R5 and the con- 
denser C3 to ground by way of one pair of contacts 
on the ODN relay. 

19.10 CHARACTERISTICS OF THE 
ELECTRONIC SYSTEM 

In order to illustrate the behavior of the two por- 
tions of the receiver system, the photographs shown 
in Figures 15 and 16 will be used. The photographs 
in Figure 15 were made using a cathode-ray oscil- 
loscope [CRO] with the sweep of the oscilloscope 
synchronized with the time base in a test set used to 
generate a signal which simulated reverberation with 
the normal rate of decay of the reverberation sig- 
nal and an echo signal superimposed on the rever- 
beration and occurring at about 180 msec after the 
beginning of the reverberation signal. The signal 
generator was so arranged that the echo signal fre- 
quency was different from the reverberation fre- 
quency by an amount which would simulate the 
doppler caused by a reasonable speed of motion of a 
target. 

In Figure 15, A shows the envelope of the rever- 
beration and echo signal which was injected into the 
receiver input circuit. The striations on the pattern 
are produced by a 60-c pickup in the oscilloscope and 
serve as a time scale on the pattern. In Figure 15, 
B was obtained by connecting the input of the oscil- 
loscope to point B in Figure 12. This is a point in the 
amplifier of the enabling receiver just preceding the 
amplitude gate. In order for the amplitude gate to pass 


reverberation signal during the initial portion of the 
listening period and to correct the local oscillator for 
own doppler of the torpedo, it is necessary to short 
the AVC by means of a contact on the ODN relay. 
In B of Figure 15 a transient can be observed after 
the fifth striation on the signal envelope. This tran- 
sient is produced when the contacts of the ODN re- 
lay are opened. It will be noted that the shape of the 
envelope from the extreme left-hand side up to this 
transient is similar to the shape of the envelope in 
the corresponding portion in A. After the AVC short 
is removed, the AVC rapidly assumes control so that 
the signal level at this point in the circuit decreases 
quite rapidly. When the echo arrives, the effect of 
the echo signal, which is at about 10 db higher level 
than the reverberation just preceding it, is to still 
further increase the AVC voltage and therefore de- 
crease the sensitivity of the amplifier. The level of 
the reverberation signal just following the echo 
measured at this point in the amplifier is actually 
lower than it is a few msec later. Figure 15C is the 
photograph of the envelope of the signal which ap- 
pears at point C in Figure 12. This is at a point fol- 
lowing the amplitude gate. The nature of the enve- 
lope of the signal during the time the AVC is shorted 
is similar to the shape of the envelope in B. However, 
when the AVC is allowed to function, the signal is 
brought down to the point where the amplitude gate 
becomes nonconducting, about 80 msec after the 
AVC short is removed. The effect of the amplitude 
gate is to reduce the signal at this point in the am- 
plifier to zero until the echo arrives. The echo is then 
passed through the amplitude gate with very little 
loss in level and, as soon as the echo has passed, the 
amplitude gate becomes nonconducting again. At D 
in Figure 15 is a photograph of the signal at point D 
in Figure 12. This signal is the doppler-enabling 
pulse. It is important to note here that, in spite of 
the fact the amplitude gate was conducting during 
about the first 160 msec of the listening interval, the 
signal which was passing the amplitude gate con- 
tained no target doppler and therefore no correspond- 
ing signal was generated at point D in Figure 12. 
However, when the simulated echo arrived, the pres- 
ence of the frequency shift due to the target doppler 
caused the pulse indicated at D to be generated. 
This is the pulse which is used to enable the steering 
amplifier in order to permit steering information to 
be developed for the steering relays. It is obvious 
from this sequence of photographs that two condi- 
tions must be satisfied by an echo. First, it is neces- 


CHARACTERISTICS OF THE ELECTRONIC SYSTEM 


101 


sary that the echo have a level above background 
reverberation in order for the echo signal to pass the 
amplitude gate. After passing the amplitude gate, it 
is necessary that this echo signal have a frequency 
different from that of the background reverberation 
in order to generate a doppler pulse for enablement 
of the receiver. The echo-to-reverberation ratio 
which is required for enablement of the system is 6 
db at a reverberation level of — 90 dbv on the trans- 
ducer and the frequency difference between rever- 
beration and echo in order to generate an enabling 
pulse from the discriminator is 60 c. 

In Figure 16 a series of photographs of oscilloscope 
patterns illustrate the behavior of the steering re- 
ceiver. These photographs were made using an oscil- 
loscope and electronic switch which made it possible 


responded to an echo from a target on the transducer 
axis, the modulation lobes would be of equal am- 
plitude. 

In C and D, of Figure 16, the signal appearing at 
point B, in Figure 11, which is the output of the 1-kc 
amplifier, is represented in the upper portion of the 
photograph. In Figure 16, C shows the signal ap- 
pearing at this point when the target is to the left 
of the axis, while D shows the signal at this point 
when the target is to the right of the axis. It is im- 
portant to note that the only difference between 
these two signals is their phase relation relative to 
the 1-kc reference signal at the bottoms of the photo- 
graphs. 

At E and F the signals appearing at the output of 
the azimuth phase-sensitive detector, which is point 


flfVPTO / r P/?£ SSUPP 

POUOW UP PfHDULUM PPUOWS 



to present two patterns on the face of a CRO simul- 
taneously. The upper pattern is the pattern produced 
by the signal which is being studied. The lower pat- 
tern is a 1-kc signal obtained from one of the ter- 
minals of the 1-kc oscillator in Figure 11. This same 
reference signal is used in all six of the photographs 
shown in Figure 16. In A, of Figure 16, the upper 
signal photograph is an envelope of a signal which oc- 
curs at point A, in Figure 11, when the signal injected 
in the input circuit corresponds to that generated in 
the transducer when an echo is received from a tar- 
get on the axis of the transducer. In Figure 16, B 
shows a photograph of a signal at the same point 
when the signal is injected only at terminals 3 and 4 
in Figure 11 and the signal corresponds to that gen- 
erated in the transducer by an echo either to the right 
or left of the axis of the transducer. If the signal cor- 


C in Figure 11, are represented. At E the signal cor- 
responds to that produced when the target is to the 
left of the transducer axis while at F the signal cor- 
responds to that produced when the target is to the 
right of the transducer axis. In this case the signal is 
in the form of rectified pulses, the difference being 
that in one case the pulses are positive, while in the 
other case the pulses are negative. In normal opera- 
tion the series of pulses occurring during receipt of 
an echo are averaged by means of the condenser C6 
in Figure 11. For the purpose of making these photo- 
graphs, C6 was removed from the circuit so that the 
individual pulses could be observed more readily on 
the oscilloscope. 

When the system is in normal operation the signal 
indicated at A and B of Figure 16 would appear, re- 
gardless of the level relative to background and re- 


102 


HARVARD UNDERWATER SOUND LABORATORY N0181 SYSTEM 


gardless of frequency difference between the signal 
and background. However, unless the requirements 
imposed by the amplitude gate and the doppler gate 
are met in the enabling channel so that a suitable 
doppler pulse is generated, the signals indicated in 
C, D, E, and F of Figure 16 would not be present 
because the enabling pulse is required in order for 
the signal to pass the last stage of the 60-kc receiving 
amplifier. 


elevators. The voltage outputs from the centers of 
these potentiometers are fed to a common point 
through the resistors Rl, R2, and R3. The relative 
values of these resistors determine the relative im- 
portance of the positions of the sliding contacts in 
the three potentiometers. The common point of the 
three resistors Rl, R2, and R3 is connected to the 
grid of the amplifier which controls the positioning 
of the vertical steering relay. 



Figure 18. Relay circuits. 


19.11 CONTROL CIRCUITS 

There are two control conditions for the torpedo. 
The first is the search condition where the torpedo 
circles in the azimuth plane and operates under hy- 
drostatic control at a fixed depth of about 225 ft. 
The direction of circling in the azimuth plane is de- 
termined by the last position of the azimuth steering 
relay. The hydrostatic control system which controls 
the depth operation uses the circuit network shown 
in Figure 17. PI, P2, and P3 are three potentiome- 
ters which are connected to the two terminals of a 
floating 45-v battery. This battery is bridged to 
ground by the two resistors R4 and R5 so that its 
center is maintained at ground potential. The poten- 
tiometer P3 is operated by the pressure bellows, the 
potentiometer P2 is operated by the pendulum, 
whereas the potentiometer PI is operated by the 


When an acoustic signal is received on the trans- 
ducer which satisfies the conditions for generating 
a pulse out of the enabling amplifier at point E in 
Figure 12, this pulse voltage is applied to grid ter- 
minal 1 of V3 in Figure 18. V3 is an amplifier which 
controls a relay called the vertical transfer relay. 
The amplifier is a two-stage direct-coupled amplifier 
in which the first stage is used as a cathode follower 
and a time constant is introduced in the output of 
this stage by means of the resistor R2 and the con- 
denser C2 so that when a signal is received on the 
grid terminal 1, the voltage generated on grid 4 will 
be held sufficiently long so that the relay will remain 
closed for about 3 sec. The contact 5 on this relay is 
connected through a 750,000-ohm resistor to grid 
terminal 1 of VI which is the vertical steering relay 
amplifier. Contact 6 on the vertical transfer relay 
connects to the common terminal of Rl, R2, and R3 


CONTROL CIRCUITS 


103 


in Figure 17. When the vertical transfer relay is open, 
which is true in the absence of echoes from a target, 
the grid of VI is connected to the output of the hy- 
drostatic control network which will then control the 
positioning of the vertical steering relay through the 
amplifier VI. When an echo is received from a target, 
the vertical transfer relay closes and contact 5 is 
connected to contact 4 which connects the grid 1 of 
Vito the vertical phase-sensitive detector. 

When the torpedo is under acoustic control, the 
steering information arrives intermittently, so that 
it is necessary that the steering relays hold in the 
position indicated by the last received echo until an 
echo arrives which indicates that the position should 
be changed. This means that under acoustic-steering 
conditions, the steering-relay amplifiers must have a 
holding feature. This feature is accomplished by 
means of contacts 5 and 6 on both the vertical and 
azimuth steering relays. When the vertical steering 
relay is in the open position, contact 6 is connected 
to contact 5 which in turn is connected to ground 
through contacts 1 and 2 of the vertical transfer re- 
lay. This means that when the steering relays are 
open, the cathodes 3 of the steering amplifiers are at 
zero potential relative to ground. When the relays 
are closed, however, the cathodes 3 of the steering 


relay amplifiers will be at a positive potential. This 
arrangement provides the holding feature for these 
relays. The holding feature for the vertical steering- 
relay amplifier operates only when the vertical trans- 
fer relay is closed. The rather long holding time for 
the vertical transfer relay causes the system to steer 
in the direction of the last-received echo in the ver- 
tical plane for about 3 sec after the last echo of a 
series is received. If no echoes are received during 
this interval, the vertical transfer relay drops open 
and the vertical steering relay is connected back to 
the hydrostatic control network which steers the 
torpedo back to the original operating depth. The 
contacts 1, 2, and 3 on both of the steering re- 
lays are the rudder control contacts. Contact 2 on 
each of these relays connects to the 36-v terminal of 
the main motor running battery. Contacts 1 and 3 
each connect to one terminal of a field winding in 
the steering motors which are provided with double 
field windings in order to make them reversible. By 
means of the holding feature in the steering relay 
systems the voltage required to change the relay 
from the closed to the open position or from the open 
to the closed position is about 2 v. a 

a See references 1-3, 6, 14, 17, 20-27, 29-36, 38-44, and 62 
for additional material on topics in this chapter. 


Chapter 20 

THE BRITISH DEALER SYSTEM 


T his device was developed on a program cor- 
responding to the NO 181 program in this coun- 
try. It was designed as an aircraft-launched, echo- 
ranging torpedo to be used against submarines with 
the gear mounted in a body 7 ft 4 in. long, 18-in. 
diameter, and weighing 590 lb. The operating speed 
is 12 knots with an operating time of 15 minutes. 


timing the transmitter, also applies the time-varied 
gain [TVG] voltage to the receiver and operates two 
sets of selector switches which permits the informa- 
tion for depth steering and azimuth steering to be 
taken from alternate pulses. The receiver is a two- 
channel amplifier followed by a phase-sensitive de- 
tector for comparing the phase of the signals sup- 



Figure 1 . Block diagram of the British Dealer system. 


The transducers and explosives are mounted in the 
head, the body contains the electronic gear, and the 
battery is mounted in the after section. A block dia- 
gram of the control is shown in Figure 1. 

The transducer assembly consists of two units : the 
first serves as the projector and the second as the 
receiver. The receiver section is divided into two por- 
tions, one for azimuth reception and the other for 
depth reception. The beam pattern is broader in 
azimuth than in depth. The transmitter, consisting 
of an oscillator driving a power amplifier, is actuated 
by a cam-operated time base which, in addition to 


plied by the two amplifiers. By means of the above- 
mentioned selector switches in the time base, steering 
in both azimuth and depth is permitted. Azimuth 
steering is accomplished by varying the relative 
speeds of two propellers which are operated by two 
separate propulsion motors. Depth steering is ac- 
complished by sliding the main running battery 
backward and forward on a set of rails by means of 
a motor-driven screw. The search plan is a descending 
helix of 50-yd radius. a 

a See reference 66 for additional material on topics in this 
chapter. 


104 


Chapter 21 

ORDNANCE RESEARCH LABORATORY PROJECT 4 SYSTEM 


21.1 INTRODUCTION 

T he results of experience with the Harvard Un- 
derwater Sound Laboratory [HUSL] device 
operating against echo repeaters and submarines in- 
dicated the desirability of making the following mod- 
ifications. 

1. The use of separate amplifiers for the enabling 
system and the steering amplifier. 

2. The use of time-varied gain [TVG] either in 
addition to or in place of automatic volume control 
[A VC]. 

3. A change in the method of securing the doppler 
notch from a circuit using a battery with no ground 
reference to an arrangement operating from a source 
of potential grounded at one end. 

4. Substitution of glide-angle control in the ver- 
tical plane for the on-off vertical steering. 

The features of the system which had proved 
especially valuable and were retained are: 

1. The transducer and input circuit which per- 
mits transmission and reception with the same trans- 
ducer without switching. 

2. The use of the quadrature-switching system 
and phase-sensitive detectors making possible the 
handling of steering information for two-plane steer- 
ing in a single amplifier. 

3. The doppler-enabling system which effectively 
prevents steering on echoes reflected from the sur- 
face and from target wakes. It also allows use of a 
higher receiver sensitivity without danger of steering 
on bursts of reverberation. 

When the program on torpedo control was moved 
to the Ordnance Research Laboratory [ORL], a re- 
search program was set up to develop a new system 
employing all the advantages of the Harvard NO 181 
system but incorporating such modifications as the 
experience gained in the Harvard NO 181 program in- 
dicated as desirable. At the same time it seemed de- 
sirable to develop this device as an anti surface-ship 
torpedo. Since there was no assignment to incor- 
porate the system into any specific existing torpedo, 
the Mark 18 was chosen as a convenient one for the 


purpose of the necessary investigations. The first in- 
vestigation in this program used a Mark 18 torpedo 
equipped with a newly designed transmitter capable 
of generating 1,500 watts of electric power and a re- 
ceiving system so set up that the self-noise level, the 
reverberation level, and the frequency spread in the 
reverberation could be measured. This device was 
designed purely as a research torpedo, with no in- 
tention of applying acoustic steering control to it. 
Since the Mark 18 torpedo is gyro-controlled and the 
beam pattern of the transducer is very narrow, a new 
system of azimuth search was incorporated. In order 
to make it possible for the torpedo to scan a rela- 
tively large angle, a special cam plate was made for 
the gyro with the cams placed 60 degrees apart. This 
causes the torpedo to operate on a snaky course such 
that the axis of the torpedo swings from 30 degrees 
on one side of the set course to 30 degrees on the 
other side. Tests run on the dynamics of this system 
at the Newport Torpedo Station indicated that the 
reduction in forward progress caused by the use of 
the ± 30-degree snaky course amounted to a little 
over 5 per cent. At the time of writing, the data 
are not complete on the reverberation level and the 
frequency spread in the reverberation. However, 
the self noise of the torpedo has been measured at 
an operating speed of 20 knots. The value is about 
5 dbs which corresponds to a signal of — 127 db re- 
ferring to 1 v [dbv] on the transducer when the re- 
ceiving band width is 4 kc. When these data are 
complete for a series of torpedo speeds, it wfill be 
possible to determine the minimum component of 
target speed parallel to the axis of the torpedo which 
will need to be used for enabling of the steering am- 
plifier at each torpedo speed. 

Figure 1 shows a block diagram of the overall sys- 
tem as worked out for project 4G. In this case the 
steering amplifier is completely independent of the 
enabling amplifier except for the enabling signal 
which is supplied to enable the demodulator. The 
amplitude gate is no longer included as part of the 
doppler-enabling system since this former arrange- 
ment caused the time constant of the amplitude gate 
to be added to the time constants of the discrimina- 


tes 


106 


ORDNANCE RESEARCH LABORATORY PROJECT 4 SYSTEM 


tor filter. The amplitude gate is incorporated as part 
of the steering amplifier and is actuated by the signal 
present in the steering amplifier circuit. In order to 
enable the system, it is necessary for a pulse to be 
developed out of the enabling amplifier at the same 
time that a pulse is developed in the steering am- 
plifier which can give rise to an amplitude gate pulse. 


transformer through a 0.01-juf condenser instead of 
to one end of the low-impedance winding. 

21.3 TIME BASE 

The time base employed in this system is a set of 
four cam-operated switches. The cams for these 
switches are mounted on a single shaft which is 



i J 


Figure 1. Block diagram of ORL project 4 system. 


21.2 TRANSDUCER AND INPUT 
CIRCUIT 

The transducer used is the same as that used in 
the Harvard N0181 system. Figure 4, Chapter 19, 
shows the pattern characteristics of a typical trans- 
ducer when high power is generated in the trans- 
mitter. These transducers have been used with a 
high-power transmitter over quite long periods and, 
although they are eventually reduced in sensitivity 
because of depolarization of the magnets, the life of 
the transducer is adequate for all practical purposes. 

The input circuit is nearly identical with that used 
in the Harvard N0181 system shown in Figure 5, 
Chapter 19. The only change is in the turns ratio of 
the input transformers, making it possible to obtain 
a voltage step-up of 30 db instead of the original 20 
db, and the 90-degree line is connected to a 1/4 tap 
on the high-impedance winding of the transmitter 


driven by the torpedo propulsion motor. Two of the 
cams operate the transmitter, the receiver blanking 
and the receiver TVG ; one is used to operate the own- 
doppler nullifier [ODN] relay and one is used as a 
range-measuring cam to control the range at which 
vertical steering takes place. Figure 2 shows sche- 
matically the sequence of events controlled by the 
time base. 

21.4 TRANSMITTER 

The schematic of the transmitter circuit is shown in 
Figure 3. VI is a 6SN7 tube, one-half of which is used 
as the transmitter oscillator and the other half of 
which is used as a buffer stage. The frequency-deter- 
mining element in the oscillator system is the induct- 
ance LI and the condensers Cl and C2. V2 is a 
6SN7 tube used as a push-pull amplifier to drive the 
grids of the driver stage V3. The grids of V2 are 
driven 180 degrees out of phase with each other by 


TRANSMITTER 


107 


means of the toroidal coil L2. V3, V4 , and V5 are all 
829B power tubes. V3 serves as the driver stage, V4 
and V5 serve as the power output stage. The trans- 
former T1 is the driver transformer used to match 


delivers 1,500 v direct current to charge a bank of 
condensers. The charged condensers serve to supply 
the 1,500 v at the center tap of T2 and 750 v for the 
screen supply for V4 and V5 as well as the plate 



Figure 2. Sequence of operations in the time base. 


the plate circuits of V3 to the grid input of V4 and 
V5. This transformer is identical with the trans- 
former T2 which is the power-output transformer 
which forms a part of the input circuit. (See Figure 


and screen supply for V3. The schematic of the 
transmitter power supply is shown in Figure 4. The 
condenser Cl is a storage condenser of 80-juf capaci- 
tance which supplies the high plate current for the 



5, Chapter 19.) The plate supply for VI and V2 is 
taken from the receiver power supply which consists 
of a 48- to 300-v motor-generator. The power sup- 
ply for V3 and V4 is a special motor generator which 


plate circuit of the transmitter during the 30-msec 
transmission interval. During the listening interval 
Cl is charged from the 1,500-v d-c generator through 
the resistor R1 which prevents an excessive load be- 


108 


ORDNANCE RESEARCH LABORATORY PROJECT 4 SYSTEM 


ing thrown on the generator during and immediately 
following transmission. 

The relay in Figure 3 and the relay in Figure 4 
are operated by cam switches in the time base. The 
relay in Figure 4 completes the circuit for the trans- 
mitter power supply and actuates the plates and 
screens of V3 and V4. The relay in Figure 3 actuates 
the plate circuit of the oscillator by connecting the 
plate to the power supply. In addition, the relay in 
Figure 3 connects the blanking voltage to the r-f 
amplifier in the steering receiver and it also connects 
the proper voltage to the TVG control circuit in the 
receiver. 


stant during the interval of the pulse. Except for the 
rounding of the corners of the pulse envelope at the 
beginning and end of the pulse, the major portion of 
the top of the pulse envelope is nearly flat. 

21.5 STEERING RECEIVER 

The steering receiver used in this device is quite 
similar to that used in the Harvard NO 181 system and 
is shown in Figure 5. The same four-channel switch- 
ing system controlled by a local 1-kc oscillator gener- 
ating signals with relative phase relations of 0, 90, 
180, and 270 degrees is used. Because of some im- 


/?/ 



Figure 4. Transmitter power supply and pulse-tailoring circuit. 


/ 500 V DC 
GENERATOR 


In order to reduce the frequency spread in the re- 
verberation, some pulse-shaping is incorporated in the 
transmitter system. This is accomplished by means 
of the inductance LI and the capacitance C3 and 
the fact that the relay in Figure 3 is not operated 
simultaneously with the relay in Figure 4. At the 
beginning of the pulse these two relays are closed by 
their respective cams at the same time. Because of 
the slight delay in build-up of the transmitter oscil- 
lator and the effect of the inductance LI in series 
with the transmitter power supply, the front of the 
pulse envelope is rounded both at the top and the 
bottom. The relay in Figure 4 is opened a few mil- 
liseconds before the relay in Figure 3 so that the 4-juf 
condenser C3 serves as the source of power to operate 
the transmitter during a short interval. This causes 
the output of the transmitter to gradually decrease, 
and it is finally stopped when the relay in Figure 3 
opens, stopping the transmitter oscillator. In addi- 
tion, the inductance Ll in Figure 4 helps to make the 
power output of the transmitter more nearly con- 


provement in design of the band-pass filter and a 
change from 6SG7 tubes to 6AK5 tubes for the 
switching tubes, the gain in the switching tubes has 
been increased from about 12 to about 30 db. The 
output of the 60-kc band-pass filter is fed to the two- 
stage amplifier VI and V2 in Figure 5. VI is a re- 
sistance-coupled stage whereas V2 has a 60-kc band- 
pass filter identical with the one in the output of the 
switching tubes for its output. The 60-kc signal with 
the 1-kc modulation is fed to the demodulator V3 
which is a grid-controlled rectifier. The enabling 
pulse from the amplitude gate and the doppler gate 
are applied to the grid of V3 which is maintained at 
a d-c potential of about — 22 v relative to the cath- 
ode. The 22- v bias on V3 is sufficient to make it non- 
conducting for any signal from the 60-kc channel. 
However, the pulse which is supplied from the dop- 
pler pulse amplifier, when added to the pulse sup- 
plied from the amplitude gate pulse amplifier, is 
sufficient to overcome this bias on receipt of an echo 
of the proper level and having the proper frequency 


STEERING RECEIVER 


109 


difference from that of the reverberation to satisfy 
the criteria set up in the enabling receiver. 

In the Harvard NO 181 device the amplitude gate 
was incorporated in the enabling receiver so that the 
time constant of the amplitude gate circuit had to be 
overcome before a signal could be applied to the dis- 
criminator driver. The time constant of the ampli- 
tude gate and the time constant in the filter of the 
discriminator were, therefore, cascaded. In this sys- 
tem the amplitude gate is operated by the signal in 
the 60-kc channel so that the time constant of the 
amplitude gate has no effect on the time of appear- 
ance of a pulse from the doppler enabling channel. 


the A VC is to prevent the device from steering on 
noise countermeasures, since it is possible for the 
noise countermeasure to generate enabling signal 
from both the doppler gate and the amplitude gate. 

Figure 6 shows the TVG-AVC and the blanking 
arrangements for the receiver. When the switch in 
the time base operates the relay in the transmitter 
indicated in Figure 3, two of these contacts in the 
relay supply voltages which are introduced in the 
TVG-AVC circuit of Figure 6 at the points labeled 
blanking voltage and TVG voltage. The blanking 
voltage used is — 48 v which is the d-c voltage avail- 
able from the power supply for operation of the fila- 


eOA'C 



Operation data obtained with the Harvard NO 181 
system indicated that, under some conditions of op- 
eration, the AVC control of amplifier sensitivity was 
not sufficient. To correct this defect, the sensitivity 
control of the stages VI and V2 in the 60-kc ampli- 
fier is furnished by a combination of TVG and AVC, 
so arranged that the AVC control can be eliminated 
by means of a switch. An important characteristic 
of a doppler-controlled device is the fact that the re- 
quirements of the sensitivity control system are not 
so rigorous as they are with a device which does not 
have the doppler-enabling feature. It is therefore 
possible to design the TVG control for operating 
conditions where the reverberation levels are a mini- 
mum and even without the use of AVC the rever- 
beration will not cause false steering. The purpose of 


ments and solenoids in the system. The TVG voltage 
which will be applied to the terminal in Figure 6 
labeled TVG voltage is a lower negative voltage, the 
optimum value of which has not yet been deter- 
mined, but a value in the neighborhood of — 10 volts 
will be used. During the time of the transmitted 
pulse a small current flow will take place through R1 
because of the difference in voltage impressed at its 
two ends. However, the condenser Cl will be charged 
to the potential of the TVG voltage and the grids of 
the 60-kc amplifier will be maintained at — 48 v 
during the transmission interval. Following trans- 
mission, the grids will very quickly take the poten- 
tial to which Cl is charged, and if the switch S is 
placed in the 1 position, the condenser will discharge 
through the 1.2-megohm resistor R2. It is the dis- 



110 


ORDNANCE RESEARCH LABORATORY PROJECT 4 SYSTEM 


charge of the condenser Cl through the resistor R2 
which produces the time variation in bias on the 
grids of the 60-kc amplifier resulting in the time vari- 
ation in gain. The A VC feature is achieved by means 


signal is received on terminal 1 of the diode from the 
60-kc amplifier, the fact that this signal charges C3 
to a potential determined by the peak value of the 
60-kc signal will determine the value of voltage-drop 


TO G/?/DS OT 


S/G/VAl 

oottut or 


J3/L/WK/S/G 
VO l TAG T 


TVG 

VOlTAGf 



Figure 6. TVG-AVC circuit. 


of the 6AL5 diode. Signal from the output of the 
60-kc amplifier is applied to terminal 1 and is rec- 
tified in the 1-7 section of the diode, charging the 
condenser C3. If the switch S is in the 2 position, the 


across R5 and will therefore affect the discharge of 
the condenser Cl. When the voltage drop across R5 
due to the rectified signal from the 60-kc channel 
reaches the value of the potential of the condenser 



Figure 7. Amplitude gate and doppler-pulse amplifier. 


condenser Cl discharges by way of the 5-2 section of 
the diode, through the resistor R4 and the resistor 
R5. The value of R4 + R5 is equal to R2, so if no 
signal is received on terminal 1 of the diode from the 
60-kc channel, the rate of discharge of the condenser 
Cl will be the same if the switch S is in the 2 position 
as it would be if S is in the 1 position. However, if a 


Cl, Cl will stop discharging and the potential on the 
grids of the 60-kc amplifier will remain constant or 
decrease at the rate of decrease of signal from the 
output of the 60-kc amplifier. The resistor R3 forms 
the completion of a path around the 2-5 section of 
the diode so that the condenser Cl can be charged 
at a very slow rate from the rectified signal from C3. 



STEERING RECEIVER 


111 


The time constant of the Cl, R3, R4 network is be- 
tween 4 and 5 sec so that a 60-kc signal of very high 
level would be necessary to produce any appreciable 
charging of the condenser Cl by way of the A VC 
network during a normal listening interval. 

Figure 7 shows the details of the entire amplitude 
gate circuit and the pulse amplifier for the doppler 
enabling pulse. VI is a 6SN7 tube, the 1-2-3 section 
of which is used to rectify a signal from the output 
of the 60-kc amplifier. The rectified signal is fed to 
the grid of the 4-5-6 section which serves as a d-c 
amplifier of the rectified voltage output of the 1-2-3 
section. It is important to note that there is no con- 
denser coupling between the rectifier and the d-c 
amplifier. This means that the output of the d-c am- 
plifier will be the same whether the signal supplied 
from the 60-kc channel is a steady signal or a pulse. 
The resistor R4 is the plate load resistor for the pulse 
amplifier portion of VI. The point PI between R5 
and R7 is maintained at a definite d-c potential in 
the absence of signal from the 60-kc amplifier which 
is determined by the fact that Rl, R2, R3, and R7 
are all returned to — 48 v and R4 is returned to the 
~b250-v power supply. 

The 1-2-3 section of V2 is a grid-controlled recti- 
fier which serves as the demodulator of the 60-kc 
signal which is applied to the cathode terminal 3. 
The d-c potential at the point Pi contributes to the 
negative bias maintained on terminal 1, the grid of the 
demodulator. The negative bias maintained on ter- 
minal 1 of the demodulator in the absence of signal 
is —22 v which is determined by the potentials of 
the points PI and P2. This negative bias on the de- 
modulator is sufficient to prevent passage of any 
signal arriving in the 60-kc channel. When a signal 
is received in the 60-kc channel, the potential of PI 
is made more positive because of the action of VI. 
If the 60-kc signal is a steady-noise signal the effect 
which it will have at PI will depend on whether the 
switch in the TVG-AVC circuit of Figure 6 is set in 
the 1 or the 2 position. If it is set in the 2 position 
and steady continuous noise is received, the A VC 
action will prevent the rise of signal in the 60-kc am- 
plifier to sufficient level to bring the demodulator 
near to the point of conducting. When an echo is re- 
ceived with the receiver under normal TVG control, 
the echo will have a level such that a pulse of voltage 
of a duration equal to the length of the echo will be 
produced at PI. This pulse of voltage will be suf- 
ficient to overcome a part of the — 22- v bias on 
terminal 1 of V2. If the echo is from a moving target, 


a pulse will be generated from the enabling amplifier 
which will be amplified in the 4-5-6 section of V2 and 
this pulse voltage, added to that generated at PI, 
will be sufficient to overcome the bias on terminal 1 
of the demodulator. The voltage capabilities of the 
two pulse amplifiers, the 4-5-6 sections of VI and V2, 
are such that one alone cannot generate a sufficient 
voltage to overcome the full bias on the demodulator 
grid. It is therefore necessary that a pulse be gen- 
erated in the enabling amplifier as well as in the 60-kc 
steering amplifier in order to overcome the bias on 
the demodulator. This arrangement achieves the 
criterion of requiring an amplitude pulse and a fre- 
quency difference between the received pulse signal 
and the reverberation in order to enable the system. 
If it is desired to operate the system as a noise-steer- 
ing device, the switch S in Figure 7 can be turned to 
the 2 position. This results in eliminating connection 
to the doppler-enabling channel and at the same 
time, by proper selection of R13, lowers the bias on 
the grid of the demodulator to a value which can be 
overcome by the potential developed at PI due to a 
noise signal. 

Figure 8 shows the schematic of the 1-kc amplifier 
and the two phase-sensitive detectors. The 1-kc am- 
plifier uses a tuned plate load and is identical with 
the one used in the Harvard N0181 system. The 
phase-sensitive detectors have been simplified some- 
what from the original Harvard N0181 arrangement. 
The new arrangement uses only one double diode for 
each phase-sensitive detector instead of two. The 
output of the 1-kc amplifier is connected to the phase- 
sensitive detectors through the condensers Cl and 
C2 and the resistors Rl, R2, R5, and R7. The acti- 
vating signals from the 1-kc switching oscillator, 
which is the same oscillator indicated in Figure 5, 
are fed to the phase-sensitive detectors through the 
resistors R3, R4, R6, and R8. The output of the 
phase-sensitive detector V2 is a d-c voltage which 
compares the signal input on the terminals marked 
UP and DN in Figure 5. If the signal on UP is larger 
in magnitude than that on DN, the d-c voltage ap- 
pearing on terminals 5 and 7 of V2 will be positive, 
or if the signal on terminal DN is larger than the 
signal on terminal UP the d-c output voltage on 
terminals 5 and 7 of V2 will be negative. The magni- 
tude of d-c voltage appearing at the output of the 
phase-sensitive detector increases with increasing 
target angle until the limiting value of the phase- 
sensitive detector output voltage is reached. This 
limiting value of output voltage is determined by the 


112 


ORDNANCE RESEARCH LABORATORY PROJECT 4 SYSTEM 


voltage of the 1-kc switching oscillator. The hori- 
zontal phase-sensitive detector V3 functions in the 
same way as the vertical phase-sensitive detector ex- 
cept that its output voltage is determined by the 
relative levels of signal on the terminals RT and 
LFT in Figure 5. 

21.6 ENABLING RECEIVER 

The enabling receiver shown in Figure 9 is similar 
in its principle of operation to the one used in the 
Harvard N0181F. It, however, uses an amplifier which 
is entirely independent of the steering receiver am- 
plifier; there is no sensitivity control and the ampli- 
tude gate is not included in it. The signal for the en- 


and the second stage uses a 7-kc tuned circuit as the 
amplifier plate load. Following this tuned amplifier 
is the discriminator driver. The discriminator driver 
is operated with a positive bias on the cathode which 
is sufficient to bias the tube beyond cutoff. The value 
of the bias used determines the level of signal on ter- 
minal 1 of the input circuit shown in Figure 5 of 
Chapter 19 which will cause a signal to be first ob- 
served in the plate circuit of the discriminator driver. 
The discriminator driver also serves as the limiter 
stage in the system. The resulting gain is such that in 
normal operation there is about a 6-db difference 
between the signal level on terminal 1 which pro- 
duces a just perceptible signal in the plate circuit of 



abling amplifier is taken from terminal 1 of the input 
circuit indicated in Figure 5 of Chapter 19. The first 
stage of amplification operates at the normal signal 
frequency of 60 kc. This stage uses an inductive load 
in the plate circuit of the tube. The output of this 
stage is coupled to the grid of the converter stage 
through a 0.0001-Aif condenser. This arrangement al- 
lows for a voltage gain from the input of the 60-kc 
amplifier to the grid of the converter stage of about 
30 db at 60 kc, while at 7 kc, which is the frequency 
to which the signal is converted, the gain from the 
input of the 60-kc amplifier to the grid of the con- 
verter is about — 8 db. All of the tubes used in the 
amplifier and converter stages are 6AK5 miniature 
pentodes. The 7-kc band-pass filter which serves as 
the plate load for the converter has been redesigned 
so that the conversion gain is considerably increased 
from that available in the Harvard NO 181 system. 

Following the converter is a two-stage amplifier. 
The first stage of the amplifier is resistance-coupled 


the discriminator driver and the signal level which 
produces complete limiting in the discriminator 
driver. This entire amplifier, from the input circuit 
to the discriminator driver, has one stage less of 
amplification than was used in the original HUSL 
NO 181 enabling system. At the same time it can be 
operated so that a signal of — 125 dbv on the trans- 
ducer will produce complete limiting of the discrim- 
inator driver. IntheHarvardN0181Fsystem, limiting 
of the discriminator driver took place at a signal level 
on the transducer of — 105 dbv. It would be possible 
to operate this amplifier so that limiting takes place 
at a signal level of — 136 dbv, if it could be used in a 
torpedo of sufficiently low noise level. 

The discriminator terminals 1 and 2 of Figure 9 
are connected to the two diodes VI and V2. By means 
of these two diodes and the diode V3, the output at 
the terminal P will be positive regardless of the po- 
larity of the signal out of the discriminator, and 
therefore, regardless of whether the echo signal has 


ENABLING RECEIVER 


113 


a higher or a lower frequency than the reverberation 
signal. The potential generated at P is amplified by 
means of the pulse amplifier which is incorporated 
in the steering receiver and shown in Figures 5 and 
7. The condensers C5, C6, C7, and C8 and the re- 
sistors R5, R6, and R7 of Figure 9 form a filter in 
one side of the discriminator circuit, while the con- 
densers C9, CIO, Cll, and C12 and resistors R8, R9, 
RIO form a filter in the other side of the discrimina- 
tor circuit. These filters are identical with the filter 


plays the role of varying the effective value of the 
condenser C14 in the tank circuit of the oscillator by 
means of the potential applied to the grid 4 of V4. 
The relay is operated by means of a cam-operated 
switch in the time base, which is so arranged that 
this relay is closed for about 50 msec immediately 
following transmission. However, in order to prevent 
sampling of the discriminator output, during receipt 
of an echo when the torpedo is near enough to a tar- 
get so that an echo is received during this 50-msec 


DISCRIMINATOR 



Figure 9. Doppler-enabling circuit. (Pin numbers of 6SN7 are 3, 1, 2, 5, 4, and 6, clockwise from lower left-hand 
cathode.) 


which was used in the discriminator circuit in the 
Harvard NO 18 IF system. The relay indicated serves 
to connect the output of one side of the discriminator 
to the grid 4 of V4, during an interval of 50 msec 
following transmission. 

The 1-2-3 section of V4 is the local 53-kc oscillator 
which supplies signal to the screen of the converter 
stage. The automatic frequency control, which ad- 
justs the oscillator frequency so that the output of 
the converter will be at the frequency of the dis- 
criminator center when reverberation is the source 
of signal, is accomplished by means of the 4-5-6 sec- 
tion of V4 with the varistor VR. This arrangement 


period, one side of the relay coil is connected to its 
source of potential through a contact on the echo 
relay which will be described in the next section. By 
arranging the switch, operated by the time base so 
that it simply connects the other side of the relay 
coil to ground, the arrangement insures that the 
relay will be closed during the entire 50-msec pe- 
riod providing no echo is received, but immediately 
on receipt of an echo signal the circuit between the 
one side of the relay coil and the 48-v power supply 
will be broken and the ODN relay will drop open. 

Since this circuit is entirely independent of other 
parts of the electronic system, except for the furnish- 


114 


ORDNANCE RESEARCH LABORATORY PROJECT 4 SYSTEM 


ing of the enabling signal to the pulse amplifier in 
the steering chassis, it is possible to completely re- 
move this enabling receiver from the other gear and 
have the rest of the device operate without doppler 
enabling providing a readjustment is made on the 


21.7 RELAY CONTROL CIRCUIT 

The details of the relay control circuit incorporated 
in the control panel are shown schematically in Fig- 
ure 10. The following four relays are used. 



Figure 10. Control circuits. 


value of the bias on the demodulator. This is pro- 
vided for by the switch shown in Figure 7, which 
makes it possible to use the steering receiver as an 
echo-ranging receiver without doppler control or as 
a noise-steering receiver in addition to its normal 
method of operation. 


1. An echo relay which is actuated during the 
time of receipt of an echo. 

2. An enabling relay which is actuated by the 
echo relay, but which remains closed for a period of 
about 6 sec following the last echo of a sequence. 

3. An azimuth steering relay which is positioned 



RELAY CONTROL CIRCUIT 


115 


by the information supplied by the azimuth phase- 
sensitive detector. 

4. The vertical steering relay which is positioned 
by the information last received from the vertical 
phase-sensitive detector. 

The following is a description of the operation and 
functions of the four relays: 

21.7.1 Echo Relay 

The echo-relay amplifier is operated by the signal 
which appears on the grid of the demodulator shown 
in Figure 5 which is normally maintained at — 22 v 
relative to ground. This potential serves as the ne- 
cessary bias for the grid of the echo-relay ampli- 
fier. The 2-5 section of the diode VI shown in the 
Figure 10 is placed in series with the grid of the echo- 
relay amplifier to provide, with the resistor Rl, a 
fast-charge slow-discharge circuit for the condenser 
Cl. Rl and Cl provide a small holding-time con- 
stant to delay somewhat the opening of the echo 
relay following receipt of an echo. This delay is 
necessary because of the fact that this relay operates 
the glide-angle control solenoid in the afterbody and 
when vertical steering takes place, it is necessary to 
hold the glide-angle control solenoid actuated for 
sufficient time to allow the glide-angle control motor 
to complete a portion of its operation. The details 
of this will be described in the next section covering 
the functions of the afterbody circuits. 

The glide-angle control solenoid is operated by 
the 2-3 contacts of the echo relay and provision for 
the removal of the ODN frequency correction, when 
an echo is received during the 50-msec ODN sam- 
pling period, is provided by the No. 1 contact. This 
is accomplished by the fact that the ODN relay coil, 
shown in Figure 9, is connected to the — 48- v supply 
via contacts 1-2 of the echo relay when this relay is 
open. Receipt of an echo immediately breaks this 
contact permitting the ODN relay to drop open. 

The 5-6 contacts, on the echo relay serve to con- 
nect the azimuth phase-sensitive detector to the grid 
of the azimuth steering-relay amplifier during the 
time an echo is being received. When the echo relay 
is open, the 5-4 contacts connect this steering am- 
plifier grid to the azimuth automatic rudder-reversal 
circuit which will be described later. Opening the 7-8 
contacts allows a 48-v surge to flow via R4 and the 
1-7 section of the diode VI to charge the condenser 
C2 and cause the enabling relay to open. The dis- 
charge of C2 through R3 and the 7-8 contacts of the 


echo relay provide the holding time required for the 
enabling relay. The components indicated in the 
figure provide a holding time of about 5.7 sec. 

21.7.2 Enabling Relay 

The normal condition of the enabling relay is 
closed. Closing of the echo relay results in grid ter- 
minal 4 of V2 being driven negative, which causes the 
enabling relay to open. The relay is held open for 
about 5.7 sec in the absence of further echoes because 
of the time constant of C2 and R3. The 1-2 contacts 
on this relay are involved in the azimuth automatic 
rudder-reversal feature to be described later. The 
contacts 4-5 connect — 48 v to the coil of a relay in 
the afterbody which sets up the circuits in the after- 
body for acoustic control. The contacts 8-9 connect 
— 48 v to the glide-angle control solenoid, when this 
relay is in the normal closed condition, in order to 
permit the glide-angle control to be returned to the 
neutral position after an attack has been broken off. 

21.7.3 Vertical Steering Relay 

The 1-2-3 section of V3 is the vertical steering- 
relay amplifier. It receives its signal directly from 
the vertical phase-sensitive detector. Contacts 1, 2, 
and 3 permit connecting one terminal of the glide- 
angle control motor to either — 48 v or ground. Since 
the other terminal of the motor is connected to — 24 
v, the position of this relay determines the direction 
of rotation of the glide-angle control motor. Opera- 
tion of the glide-angle control motor is determined 
by the echo relay, the enabling relay, and a range- 
measuring cam in the time base. The details of these 
circuit arrangements will be described in the next 
section. The 5-6 contacts on the vertical steering re- 
lay provide a holding feature by grounding the junc- 
tion of Rll and R12 when the relay is closed. By 
means of this holding feature the relay can be made 
to open on a — 2-v signal on the grid of the amplifier 
and close with a +2-v signal on the grid and it will 
retain its last position until a suitable signal arrives 
to change it. 

21.7.4 Azimuth Steering Relay 

The azimuth steering-relay amplifier is connected 
to the azimuth phase-sensitive detector by way of 
the 5-6 contacts of the echo relay. This relay ampli- 
fier has the same sensitivity and holding feature as 
is incorporated in the vertical steering relay with the 
8-9 contacts on the relay used to control the holding 


116 


ORDNANCE RESEARCH LABORATORY PROJECT 4 SYSTEM 


feature. The automatic rudder reversal feature of the 
azimuth steering system is arranged through the 
contacts 1, 2, and 3. The 4, 5, and 6 contacts are the 
steering contacts which provide for — 48 v to be con- 
nected to one or the other of the azimuth steering 
solenoids. The ground return for these solenoids is 
provided through the relay in the afterbody which 
is operated by the enabling relay. Before the first 
echo is received, it is impossible for the solenoids to 
be operated by the azimuth relay. When an echo is 
received, the closing of the enabling relay provides 
for connection of the ground return from the solenoid 
so that the azimuth steering relay takes over control 
of the azimuth steering from the gyro. The gyro lock- 
off feature in the afterbody provides for permanent 
ground return of the solenoids after a series of about 
ten echoes has been received. When this has been 
accomplished the azimuth steering relay will retain 
control of the azimuth steering regardless of whether 
echoes are being received or not. 

21.7.5 Azimuth Automatic Rudder- 
Reversal Feature 

Assume that the torpedo is steering to starboard, 
the azimuth rudder relay is down, and the torpedo is 
in an attack. Under these conditions — 48 v are con- 
nected through the 3-2 contacts of the azimuth steer- 
ing relay and the 2-1 contacts of the enabling relay, 
and R5 and R2 to the terminal of the condenser 
C3. While the echo relay is open, C3 is connected 
through the 4-5 contacts of the echo relay to the 
grid of the azimuth steering relay. When the con- 
denser C3 arrives at a sufficiently negative potential 
( — 2 v) the azimuth steering relay will be thrown 
open, causing the torpedo to steer to port. The same 
sequence would be carried out if the azimuth relay 
had originally been open except that, in this case, 
terminal 2 of the azimuth relay would be connected 
to the bleeder circuit R18 and It 19 which supplies a 
positive potential with which to charge C3, which 
would cause the relay to close when C3 arrives at a 
potential of +2 v. The action of the 8-9 contacts on 
the echo relay is to discharge C3 through R2 each 
time an echo is received so that the rudder reversal 
can take place only when an attack breaks off. Since 
C3 is charged through the 2-1 contacts of the en- 
abling relay the rudder reversal must take place dur- 
ing the holding time of this relay. Since only one re- 
versal is desired when an attack breaks off, the time 
constant of the rudder-reversal circuit which is de- 


termined by C3, R5, and R6 must be more than half 
the holding time of the enabling relay, and to insure 
the reversal taking place, the time constant must be 
less than the holding time of the enabling relay. 

21.7.6 Sequence of Events in an 
Attack when an Echo is Received 

The echo relay drops closed and remains closed 
for several milliseconds after the end of the echo. 
The azimuth and vertical steering relays take up 
settings which are determined by the outputs of the 
azimuth and vertical phase-sensitive detectors. The 
vertical steering relay will not be able to exert control 
on the glide-angle control system unless the range is 
less than 250 yd, since the actuation of the glide- 
angle control solenoid by the echo relay is accom- 
plished by way of a cam-operated range switch in the 
time base. The azimuth relay will immediately take 
control of the azimuth steering from the gyro and 
the gyro lock-off motor will start. Three separate 
possible cases will be considered. The first possibility 
is that only one or two echoes are received and the 
range is greater than 250 yd. Under this condition, 
no vertical steering takes place. After the last echo, 
the azimuth rudder relay will hold for about 4.5 sec, 
when the rudder will reverse, causing the torpedo to 
start to recross the beam. However, at the end of 
about 6 sec following the last echo, the enabling relay 
will drop closed and return the torpedo to gyro con- 
trol. The second possibility is that a series of ten or 
more echoes are received so that the gyro lock-off 
motor has completed its cycle. The action will be the 
same as before, except that after rudder reversal the 
torpedo will continue to circle in the direction deter- 
mined by the reversed-rudder relay position. The 
third possibility is that a series of echoes have been 
received for a sufficient time to cause the gyro lock-off 
to complete its cycle and the attack has carried the 
torpedo into the range where vertical steering can 
take place. The glide-angle control functions to allow 
the torpedo to correct its course by about 1.5 degrees 
in the vertical plane each time an echo is received 
within the 250-yd vertical steering range. If the tor- 
pedo loses contact with the target, it will continue to 
climb at the climb angle determined by the last set- 
ting of the glide-angle control until the enabling relay 
drops closed; i.e., until about 6 sec after receipt of 
the last echo. After this, the glide-angle control will 
be returned to its initial neutral position by one in- 
crement of angle in each ping interval. In the mean- 


CIRCUITS IN THE TORPEDO AFTERBODY 


117 


time, the azimuth rudder reversed after a lapse of 
4.5 sec from the last-received echo. Since the glide- 
angle control return takes place in increments, the 
torpedo will have a chance to reverse its course and 
sweep across the beam before it has a chance to re- 
turn to its hydrostatic running depth. If the loss of 
contact occurs at a point very close in, there is a 
chance that the automatic rudder-reversal feature 
will cause the torpedo to strike the target in spite of 
loss of acoustic contact. 


21.8 CIRCUITS IN THE TORPEDO 
AFTERBODY 

Figure 11 shows the circuit arrangements for the 
afterbody of the torpedo. The circuits are arranged 
for the use of generator power supplies located in the 
afterbody. The time base is operated as a system of 
cams driven by the main motor shaft and is indicated 
in the figure as the cams Cl through C4. The azi- 



The time constants given for the automatic rudder 
reversal, the holding of the enabling relay, and the 
gyro lock-off are not necessarily the values which will 
eventually be used, since their value will have to be 
determined by the body dynamics of the torpedo. In 
order to prevent broaching of the torpedo when it 
loses contact close in, it will probably be necessary 
to have a ceiling switch in the afterbody which will 
permit immediate return to hydrostatic control if 
acoustic contact is lost at depths less than 10 ft. 


muth steering solenoids are arranged so that they 
take over control of the steering from the gyro when 
they are actuated by the relay circuits. The relay 
designated as relay 2 in Figure 11 is closed by the 
48-v power supply when the power supplies are 
turned on. This is provided in order to permit re- 
cycling of the glide-angle control system and the 
gyro lock-off system after prerun tests have been 
completed. The following is a description of the var- 
ious components and their functions. 



118 


ORDNANCE RESEARCH LABORATORY PROJECT 4 SYSTEM 


21.8.1 Provision for the Gyro Lock-off 

The gyro lock-off consists of a small 24-v perma- 
nent-magnet motor which drives cams Cl and C2 
through a reduction gear. In the figure, the arrows 
on the motor and cams indicate the direction of ro- 
tation of the system when it is progressing toward 
the gyro lock-off condition. At the start, cam Cl is 
in such a position that it holds the switch Si open. 
When the first echo arrives, closing of the enabling 
relay on the main chassis actuates relay 1 causing it 
to close. This connects brush 1 on the gyro lock-off 
motor to — 48 v. Brush 2 on the motor is connected 
via contacts 3 and 4 of the switch S2 and contacts 
5-6 of relay 2 to — 24 v. The motor continues to ro- 
tate in the forward direction as long as relay 1 re- 
mains closed or until cam C2 opens switch S2. If the 
attack breaks off long enough to allow the enabling 
relay to open, relay 1 opens and brush 1 of the gyro 
lock-off motor is connected to ground by way of con- 
tacts 4-5 on relay 1 and contacts 1-2 on the switch 
SI. This causes the motor to reverse and it continues 
to rotate in the reverse direction in the absence of 
further echoes until cam Cl opens switch Si breaking 
the motor circuit. Spurious echoes will start the gyro 
lock-off motor, but as soon as the enabling relay 
drops open, the motor will return the gyro lock-off 
system to the original starting position. The time 
which is set up for gyro lock-off to be accomplished 
is about 15 sec, corresponding to the time necessary 
for about ten consecutive echoes to be received. The 
lock-off feature is achieved by means of the cam C2 
which, when turned far enough, opens switch S2, 
which breaks the circuit to the brush 2 on the gyro 
lock-off motor from the —24-v supply. 

21.8.2 Glide-Angle Control System 

The glide-angle control system is the means used 
to transfer the information from the vertical steering 
relay to the elevators. This is done by adding incre- 
ments of mechanical bias to the pendulum in the im- 
mersion mechanism. The bias is added to the pen- 
dulum in such a way that it is compensated for by a 
tilt of the torpedo body through a definite number 
of degrees. The amount of bias added to the pendu- 
lum on each received echo is about 1.5 degrees. 

It is desired that vertical steering be restricted to 
a target range of about 250 yd or less. This is accom- 
plished by the use of cam 4 in the time base which 
keeps its switch closed during the time that an echo 
could be received from a target range of 250 yd or 


less. If an echo is received within this range, the echo 
relay in the panel will be closed which connects — 48 
v, via terminal D on the large AN plug, terminal E 
on the small AN plug, the switch operated by cam 4 
and the 11-12 contacts on relay 2 to the glide-angle 
control solenoid. This solenoid is the coil of a relay 
which, when it is actuated, closes contacts 1-2, while 
the extended arm on the relay is pulled away from 
the disk D. This arm has a pin which fits into holes 
around the edge of the disk D so that the contacts 
1-2 are held closed, except when the pin is free to 
drop into a hole in the disk. Closing the contacts 1-2 
connects brush 1 of the glide-angle control motor to 
— 24 v through the 2-3 contacts of relay 2. Brush 2 
of the motor is connected via the 2-3 contacts of 
relay 1 and the AN plugs to a swinger on the vertical 
steering relay in the panel. When the phase-sensitive 
detector signal indicates up-steering, this brush will 
be connected to ground, whereas if it indicates down- 
steering, this brush will be connected to — 48 v. The 
direction of rotation of the motor, therefore, is de- 
termined by the setting of the vertical steering relay. 
As soon as the echo relay drops open after the end of 
an echo, with the enabling relay still in its actuated 
condition, the glide-angle control solenoid will be un- 
actuated and it will try to return to normal. How- 
ever, in the meantime, the disk D will have turned 
enough so that the pin on the extended relay-arm 
will ride on the surface of the disk between holes, 
thus holding the contacts 1-2 closed until the next 
hole on the disk comes into position so the pin can 
drop in and open the contacts. This arrangement in- 
sures that the motor will turn the disk through an 
angle determined by the location of adjacent holes 
each time an echo, permitting vertical steering, is 
received. A small holding time is incorporated in the 
echo relay to make certain that the glide-angle sole- 
noid may remain actuated for sufficient time so that 
the pin on the extended arm cannot drop back into 
the same hole from which it is pulled out. 

If an attack is broken off, the enabling relay will 
eventually drop open. When this occurs, the relay 
setup on the steering panel will be such that the 
glide-angle control solenoid will be actuated during 
each interval that the switch operated by cam 4 in 
the time base is closed. If some bias has been intro- 
duced on the pendulum by means of the glide-angle 
control system, contact 2 in the switch which is op- 
erated by the mechanical bias rack will be closed 
either against contact 1 or 3 depending on whether 
the bias introduced is for up or down steering. When 


CIRCUITS IN THE TORPEDO AFTERBODY 


119 


the enabling relay in the steering panel is unactuated, 
relay 1 in Figure 11 will open and brush 2 on 
the glide-angle control motor will be connected 
via contacts 2-1 pn relay 1 to contact 2 of the 
switch operated by the mechanical bias rack. The 
polarity of contacts 1 and 3 is such that the motor 
will be driven in a direction so that the mechanical 
bias rack will be returned to its neutral position. 
Because of the intermittent operation of the glide- 
angle control solenoid under this condition, the glide- 
angle control bias will be returned by one of the 
increments of bias during each ping interval. When 
the bias rack has been returned to the neutral posi- 
tion, contact 2 on the bias-rack switch will be in a 
free position between contacts 1 and 3, thus breaking 
the motor circuit. In order to prevent broaching of 
the torpedo when acoustic contact is lost near the 
end of an attack, a pressure-operated ceiling switch 
is placed in parallel with the switch operated by cam 
4. This permits the glide-angle control motor to re- 
turn the pendulum bias to the neutral condition 
immediately on opening of the enabling relay if the 
torpedo is at a depth shallower than the setting of 
the ceiling switch. 

The provision for recycling of the gyro lock-off and 
the glide-angle control is made by means of the bat- 
tery B and the change of connections provided when 
relay 2 is unactuated by turning off the source of 
48-v power. If the gyro lock-off has completed its 
cycle so that S2 is open, brush 2 of the gyro lock-off 
motor will be connected to the 24-v terminal on the 


battery B by way of contacts 4-5 or relay 1 and con- 
tacts 1-2 of switch 1. This will permit the motor to 
start to recycle even though the gyro lock-off motor 
has turned sufficiently to allow the switch S2 to open. 
Brush 2 of the motor will be connected to the — 24-v 
terminal of the battery B by way of contacts 4-3 of 
the switch S2 and contacts 4-5 of relay 2 so the motor 
will continue to recycle until cam 1 opens the switch 
SI which is the condition for starting of the system. 

The glide-angle control solenoid is connected to 
the -48-v terminal of the battery by way of con- 
tacts 10-11 of the relay 2 and the 4-5-6 contacts 
operated by the mechanical bias rack providing the 
glide-angle control is not in the neutral position. 
This will cause the 1-2 contacts operated by the 
glide-angle control solenoid to be closed and to re- 
main closed until the mechanical bias rack is re- 
turned to the neutral position. Brush 2 of the glide- 
angle control motor will be connected to either ground 
or the —48-v terminal B by the mechanical bias- 
rack switch, if it is not in the neutral position and 
the polarity determined by contacts 1, 2, and 3 
are such that the rack will be run back toward the 
neutral position. As soon as the neutral position is 
reached, the motor circuit will be broken by the 
1-2-3 contacts in the mechanical bias-rack switch 
stopping the motor. At the same time the connection 
to the glide-angle control solenoid will be broken by 
the 4-5-6 contacts on this switch. a 

a See references 6, 11-13, 15, 16, 18-20, 26, 27, 29-32, 37-47 
for additional material on topics in this chapter. 


Chapter 22 

BELL TELEPHONE LABORATORIES 157B AND 157C SYSTEMS 


22.1 INTRODUCTION 

T he original Bell Telephone Laboratories [BTL] 
development was an echo-ranging system to be 
used in the submarine-launched Mark 14 torpedo 
and was known as Project 157B. However, when the 
use of the Mark 18 electric torpedo began to super- 
sede the use of the Mark 14, BTL was asked to mod- 
ify their system for use in the Mark 18 torpedo. The 


ing transmission, so that equal acoustic signals, in 
phase with each other, are transmitted into the 
water. The receiver consists of two similar amplifiers, 
with each amplifier connected to the output of one- 
half of the transducer. At the output of each ampli- 
fier channel is a threshold stage which imposes the 
requirement that a signal level developed in the 
transducer must exceed some predetermined value 
as a function of time after transmission in order to 



Figure 1. Block diagram of 157C system. 


work on the adaptation of the system to the Mark 
18 was being concluded at the end of World War II, 
and while no tests had been run on this modified de- 
vice, some units, designated as 157C, were about 
ready for field operations. Since the only essential 
differences between the 157B and 157C systems are 
those necessary to adapt the systems to the two dif- 
ferent torpedoes, the description in this report will be 
confined to the one used in the Mark 18 torpedo, 
the 157C. 

A block diagram of the system is shown in Figure 
1. Acoustic control is confined to the azimuth plane 
with the normal hydrostatic control in depth. The 
transducer is divided into two halves in the azimuth 
plane with the two halves connected in parallel dur- 


pass this threshold stage. The time variation in signal 
level which is imposed is controlled by a time-varied 
gain [TVG] control on the receiving amplifiers. Fol- 
lowing threshold stages are limiter stages. The char- 
acteristics of the threshold and limiter stages are such 
that about 2 db of signal level above that necessary 
to pass the threshold stage is sufficient to limit the 
limiter stage. This assures that a signal level, a very 
little higher than the minimum permitted to pass 
the threshold stage, will result in a level out of the 
limiter stage which is independent of received signal 
level. The outputs of the two limiter stages are fed 
to a phase-sensitive detector. The phase-sensitive 
detector measures the electrical phase relation be- 
tween the signals which have been supplied by the 


120 


TRANSDUCER 


121 


two halves of the transducer, and from this electrical 
phase difference the target bearing relative to the 
axis of the torpedo is determined. The information 
determined by the phase-sensitive detector is sup- 
plied to a device known as a translator. The trans- 
lator is a mechanical system w T hich is connected to 
the cam plates on the torpedo gyro. The correction 
to be applied to the course of the torpedo, as deter- 
mined by the phase-sensitive detector, is applied di- 
rectly to the gyro cam plate by the translator. In 
this way, the torpedo remains under gyro control for 
the entire course of the run; but, after an attack be- 
gins, the system is able to make a correction on the 
setting of the gyro course of the torpedo on each 
received echo. 

It is necessary to make provision in the phase- 
sensitive detector system against the torpedo homing 
on the wake. This is done by means of a preferred- 
side steering. At the time the torpedo is fired, the 
side of the target on which it is being fired is deter- 
mined and a switch operable from the outside of the 
torpedo body is set so that, if the torpedo is fired 
toward the port side of a target, the system will pre- 
fer echoes reflected from the point on the target 
furthest to port. This means that when an echo is 
received from an extended target including the full 
length of a ship and its wake, the torpedo will home 
on the bow end of the entire system. 

The transmitter supplies about 1,500 watts of elec- 
tric power to the transducer. The transmitted pulses 
are 3 msec in length and they are spaced at 1-sec 
intervals. This spacing of the transmitter pulses 
makes the maximum acoustic range of the torpedo 
about 800 yd. The frequency at which the system 
operates is about 28 kc. 

22.2 TRANSDUCER 

Figure 2 shows the details of the elements of the 
transducer used. It consists of an array of piezo- 
electric crystals mounted on a steel resonator plate 
which is enclosed in a chamber formed by the resona- 
tor plate and a thin steel dome. The crystals are 
ammonium dihydrogen phosphate 45-degree z cut. 
Twenty-four crystals are used with eight of them 
half-amplitude in order to obtain a system which 
suppresses minor lobes. The crystals are separated 
from the steel plate by means of ceramic insulators. 
The crystals are cemented to the ceramic insulators 
and the ceramic insulators are in turn cemented to 
the resonator plate. The space between the resonator 


plate and the thin steel dome is filled with Union 
Carbide and Carbon Corporation’s UCON HB 600 
fluid. 



Figure 2. Transducer crystal array. 


The steel dome is 0.030 in. thick and mechanical 
reinforcement is provided by an internal grill work 
of 3/16-in. stainless steel rods. Figure 3 shows details 
of the dome. Figure 4 shows the directivity patterns 



Figure 3. Transducer dome. 

for this transducer in both the horizontal and azi- 
muth planes and for both transmission and recep- 
tion. Figure 5 shows the electrical phase angle be- 
tween the signals generated in the two halves of the 
transducer measured as a function of the angle of 


122 


BELL TELEPHONE LABORATORIES 157B AND 157C SYSTEMS 



TRANSMITTING 



RECEIVING 


Figure 4. Directivity patterns of transducer. 


the incident acoustic signal. This is an important 
characteristic since the system measures the bearing 
of the target relative to the axis of the torpedo by 
comparing the phase angle of the signal generated in 
the two halves of the transducer. 

22.3 TIME BASE 

The operation of the torpedo is based on the use of 
3-msec pulses transmitted at 1-sec intervals, making 
the maximum possible acoustic range of the torpedo 
a little over 800 yd. During transmission, transients 
are induced in the tuned circuits of the receiver which 
persist for a short time following transmission. In 
order to prevent false steering on these transients 


4 00 

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ANGLE OF SOUND INCIDENCE 

Figure 5. Electrical phase angle plotted as a function 
of target angle for the hydrophones. 

the receiver is blanked for a total of 40 msec begin- 
ning at the time of start of transmission. Figure 6 
indicates diagrammatically the sequence of operations 






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TIME BASE 


123 


controlled by the time base. The time base consists 
of a multivibrator which actuates relays during its 
on period. A schematic of the entire time-base system 
is shown in Figure 7. The normal off period of the 


by which the end of Cl connected to terminal 3 of 
V2 is rapidly charged to approximately +300 v dur- 
ing the 50-msec on period. Since this condenser has 
a large capacity (1 /if), a low-impedance charging 



-1600 MILLISEC 



Figure 6. Sequence of events in the time base. Note: Dimension “1600 millisec” should read 1000 millisec. 


multivibrator is 950 msec and its on period is ap- 
proximately 50 msec. Timing of the off period is 
determined by Cl, Rl, and PI. Adjustment of the 
off period is accomplished by means of the poten- 


path is required to provide full charging of this con- 
denser. This charging path is provided by the 1-2-3 
section of the tube V2 acting as a cathode follower. 
The normal operation of the panel requires that 


BOTTOM RC ros* 7T>r ST BOTTOM 



tiometer PI. The on period is controlled by C2, R3, 
and P2, when the latter two are connected to the 
+300-V supply by the 5-6 contact of the relay RC. 
An unusual feature of the multivibrator is the circuit 


the reverberation control relay RC shall be closed 
for 40 msec. The multivibrator circuit is designed to 
insure stability of this requirement. The operation is 
as follows: When the 2-3 section of the tube VI be- 




124 


BELL TELEPHONE LABORATORIES 157B AND 157C SYSTEMS 


gins conducting, terminal 4 is driven negative and 
current from terminal 5 to 6 ceases. The resultant 
flow of current from terminal 5 to 6 of V2 actuates 
the starting relay ST which in turn actuates the 
relay RC. During this period, the voltage on ter- 
minal 4 of VI has risen only slightly because of the 
high resistance of R4. When the contacts on the re- 
verberation control relay RC are closed, R3 and P2 
are returned to +300 v and the time constant of 
these resistors in combination with the condenser C2 
determines the remaining time during which ter- 
minal 4 of VI remains negative and, therefore, the 
time until current in the starting relay coil ST is 
interrupted. In order to make the operated period 
of the reverberation control relay RC 40 msec, the 
time constant of the system R3, P2, and C2 is made 
less than 40 msec by the closing time of the relays. 
This is accomplished by adjusting P2 until the op- 
erated period of the reverberation control RC relay 
is 40 msec. 

In order to control the length of the transmitter 
pulse, the multivibrator consisting of the tube V3 
and its associated components is used. A common 
cathode resistor R13 is employed to complete the 
feedback loop. The multivibrator exercises its control 
by applying a positive pulse to the screen of the 
transmitter tube through a cathode-follower stage. 
During the off period of this multivibrator, the plate 
current from terminal 5 to 6 is determined by the 
choice of tube type, the plate-load resistance R16 
and the common cathode resistance R13. To drive 
the plate as near —300 v as possible, the voltage 
drop across R13 should be as low as possible, while, 
to maintain the bias on the 1-2-3 section of the tube 
sufficiently below cutoff to prevent false triggering, 
this voltage drop should be as high as possible. A 
workable compromise is achieved by the use of a 
6SN7 tube with the values assigned to the plate and 
cathode resistors indicated in Figure 7. Triggering 
of this multivibrator is accomplished by a contact 
on the relay RC in the following manner. Between 
pulses, the left-hand end of C4 is charged to — 105 v 
through the 100-megohm resistor R10. Closing the 
bottom contacts 1-2 of the relay RC grounds this 
point and produces a positive pulse on terminal 1 of 
V3 of approximately 18 fisec duration and 105 v 
magnitude. Chatter of the relay contact in RC does 
not tend to cause false triggering due to the long 
time required to recharge the condenser C4. The 
3-msec on period of this multivibrator is determined 
primarily by C6 and R15. 


22.4 TRANSMITTER 

The transmitter consists of a self-excited class C 
oscillator which is keyed by applying a 3-msec posi- 
tive pulse to its screen by means of the 3-msec mul- 
tivibrator and a cathode-follower stage. A schematic 
of the transmitter circuit is indicated in Figure 8. 
The plate supply for the oscillator is 3,000 v ob- 
tained from a 7.5-/xf storage condenser which is 
charged during the intervals between transmitted 


TO 

TMrtSDUCe/? 



pulses by means of a high-voltage transformer and 
rectifier. The plate current of the oscillator tube V2 
is approximately 1.33 amp during the 3-msec interval 
of transmission. The voltage across the storage con- 
denser is reduced by approximately 20 per cent dur- 
ing this interval. 

The frequency-determining element of the oscil- 
lator is a polystyrene-insulated condenser C4 and 
the inductance T2 which is tuned by means of a 
movable permalloy slug. Frequency stability is ob- 
tained by close coupling of the grid and cathode 
windings in the oscillator coil, and is further assured 
by the fact that the high plate resistance of the 
tetrode V2 is between the load and the frequency- 
determining element of the circuit. 

During the off period, VI is nearly nonconducting 
and the screens of V2 are biased at approximately 
— 80 v. In practice it is necessary to do some select- 
ing of the 3E29 tubes which are used for V2 in order 
to insure low enough plate current drain during the 


TRANSMITTER-RECEIVER SWITCHING CIRCUIT 


125 


off period to permit the storage condenser to be 
charged to the proper voltage. 

During the transmitting period, the 3-msec multi- 
vibrator applies a +300-V pulse to the grid of VI 
which, acting as a. cathode follower, supplies a pulse 
of about +250 v to the screen of V2. In the design 
of the oscillator, three factors had to be considered: 
frequency stability, time of pulse build-up and decay, 
and tank-circuit efficiency. An extremely high-Q 
tank circuit results in high frequency stability and 
high tank-circuit efficiency but slow build-up time. 
However, the rate of build-up is also determined by 
the excess gain in the oscillator feedback loop. The 
constants for this oscillator were chosen as a com- 
promise between these factors such that build-up and 
decay times of approximately 0.5 msec are realized. 
The resulting frequency stability is adequate to meet 
the overall requirements of the system. 

22.5 TRANSMITTER-RECEIVER 
SWITCHING CIRCUIT 

Since a single transducer is used for the functions 
of both projector and hydrophone, it is necessary to 
provide means of switching between transmission 
and receiving in order to prevent the receiver from 
being damaged during transmission and to prevent 
the transmitter from providing a load on the trans- 
ducer during reception. Figure 9 shows a schematic 
of the switching system used, which consists of four 
thyrite elements, RV1, RV2, RV3, and RV4. These 
thyrite units have an impedance characteristic which 
varies in inverse proportion to the third power of the 
current passing through them. Thus RV3 and RV4 
become low-resistance elements under the influence 
of the transmitter during the 3-msec transmitting 
interval and, therefore, serve to connect the trans- 
mitter transformer to the two transducer units con- 
nected in parallel. The voltage drop through the 
thyrite units during transmission is of the order of 
100 v and, therefore, a negligible power loss is in- 
troduced. During the receiving interval when the 
signal voltages developed by the hydrophones are 
low, RV3 and RV4 become high-resistance elements 
and therefore serve to isolate the two halves of the 
transducer from the transmitter. In practice, be- 
cause of the capacity of the thyrite units, their im- 
pedance cannot be considered to be so high as to 
have no effect on the receiving circuit. They provide 
a certain amount of coupling between the transmitter 
output transformer and the hydrophones, particu- 


larly if the capacities of the two halves of each thy- 
rite unit are unequal. This causes noise, originating 
in the transmitter transformer caused by either in- 
ductive pickup or ripple on the power supply, to be 
fed into the receiver. This factor is a serious limita- 
tion on the entire system since this source of noise 
may be the limiting factor in receiver sensitivity 
rather than the torpedo self noise. 

The voltages applied to the transducer during the 
transmitting pulse would be damaging to the re- 
ceiver input transformers if they were not prevented 
from reaching these elements. Isolation is provided 
for the starboard receiving channel by condensers Cl 
and C2 and the thyrite unit RV1 and for the port 


port 

r RAN5 OUCfR 



output 


Figure 9. Thyrite switching system. 


receiving channel by the condensers C3 and C4 and 
the thyrite unit RV2. During the time of transmis- 
sion the thyrite units RV1 and RV2 become low 
impedance and therefore limit the voltage applied to 
the primaries of the receiver transformers to approxi- 
mately 275 v. This is a sufficiently high magnitude 
to initiate transients in the entire receiver circuit, in 
spite of the presence of a blanking bias on the grids 
of the first two stages of the receiver. It is, however, 
sufficiently low to protect the transformers from in- 
sulation breakdown. During the receiving interval 
RV1 and RV2 are high-resistance elements, but they 
represent, nevertheless, shunt capacities across the 


126 


BELL TELEPHONE LABORATORIES 157B AND 157C SYSTEMS 


primary of the receiver transformers. These in com- 
bination with additional condensers are used to pro- 
vide tuning for the primaries of the receiver trans- 
formers. The condensers Cl, C2, C4, and C5, in 
combination with the shunt elements including the 
transmitter transformer winding constitute an at- 
tenuating network providing a voltage loss of ap- 
proximately 13 db. 

22.6 RECEIVING AMPLIFIERS 

The port and starboard halves of the transducer 
each connect to a separate receiving amplifier by 
means of a tuned coupling transformer. The am- 
plifiers are conventional with three stages. The inter- 
stage coupling and the output coupling are accom- 
plished by means of band-pass transformers. These 
transformers are so designed that the gain is 3 db 
below maximum at ± 800 cycles from the nominal 
mid-band frequency of 27.75 kc. This band width is 
chosen to accommodate three factors: first, frequency 
shift due to doppler effect caused by motion of the 
target; second, the band width necessary to pass a 
3-msec echo pulse with negligible envelope distor- 
tion; and third, the band width necessary to accom- 
modate possible drift in the oscillator frequency. 

During transmission, high-level transients are gen- 
erated in the tuned circuits; however, these drop to 
less than expected signal levels by the end of the 40- 
msec blanking period. 

The first two stages of the amplifier are variable- 
gain stages with the gain variation achieved by var- 
iation of the grid bias on the tubes. 

The steering control is determined by comparing 
the electrical phase relation between the signals gen- 
erated in the two halves of the transducer. The 
phase-sensitive detector which makes this compari- 
son follows the amplifiers, so it is important for the 
electrical phase shift introduced in the two amplifier 
channels to be the same. A tuning condenser is pro- 
vided in one channel to provide an adjustment to 
satisfy this condition. 

22.7 THRESHOLD CIRCUIT 

The threshold circuit, shown schematically in Fig- 
ure 10, is identical in the two channels. In operation, 
terminals 4 and 5 of the diode are maintained at 
— 52.5 v by the voltage divider R1 and R2 which is 
fed from a — 105-v regulated power supply. Terminal 


3 of the diode is maintained at — 105 v while terminal 
8 is at ground potential. With these electrode po- 
tentials, no conductive current flows through either 
section of the diode for signals which have peak volt- 
ages less than 52.5 v. However, if a signal has a peak 
value greater than 52.5 v, the portion of the positive 
peak at terminal 1 of T1 which is greater than 52.5 
v will cause current to flow from terminal 5 to ter- 
minal 8 of the diode. Similarly, that portion of the 
negative peak which is greater than 52.5 v will cause 
current to flow from terminal 3 to terminal 4 of the 
diode. Since these currents are equal there is no 
change in the direct currents in R1 and R2, so their 


P?EC£ r /U£ft 

amplifier 

ourpuT 



Figure 10. Threshold stage circuit. 


junction will remain constant at — 52.5 v even in the 
presence of a signal. The current pulses which pass 
through the two diode sections also pass through 
corresponding windings of T2 which are so wound 
that an alternating voltage of the same frequency as 
that in T1 is induced in the 1-2 winding of T2. Be- 
cause of the intermittent current flow, there will be 
considerable harmonics induced in this winding. 

The condenser C3 is used to reduce the value of 
the voltage at T2, caused by capacity unbalance in 
the diode and its associated wiring, to a minimum 
when the signal voltage on the secondary of T1 is 
less than 52.5 v. 


VARIABLE GAIN-CONTROL FOR THE RECEIVER AMPLIFIER 


127 


22.8 VARIABLE GAIN-CONTROL FOR 
THE RECEIVER AMPLIFIER 

The function of the variable gain-control circuit 
is to adjust the gain of the first two stages of the 
amplifier so that the level of output of the amplifier 
caused by noise and reverberation will be brought to 
a fixed level below the threshold. By so doing, any 
signals are amplified as much as possible without 
danger of reverberation or self-noise signals passing 
the threshold stage. Since the signal is composed of 
reverberation, which is a decaying factor, and self 


ence of signal at the output of the third amplifier 
stage is sufficient to assume control. For the re- 
mainder of the 1-sec listening interval, the gain vari- 
ation is under the control of the signal output of T9 
which is a winding in the output transformer of the 
third stage of each amplifier. 

The receiver gain-control is separate for the right 
and left receiver channels. A schematic of the circuit 
used is shown in Figure 11. For times less than 32 
msec after transmission, corresponding to a range 
of 24 yd, the sensitivity of the amplifier is essentially 
zero because of the operation of the relay ST. For 



noise, which is a relatively steady factor, the gain- 
control circuit is set up so that the gain is always in- 
creasing or held constant; i.e., it cannot decrease 
during a listening period. 

The gain control should not be confused with the 
disabling of the circuit during and shortly after 
transmission. While the gain is held to a minimum 
during the disabling period, the actual disabling is 
accomplished by the relay ST, indicated in Figures 7 
and 10, at a point further on in the circuit. 

At a time 40 msec after transmission, the gain of 
the receiver begins to increase under the control of 
the gain-control circuit, rising rapidly at first at a 
rate of approximately 24 db for double time inter- 
vals and continuing to rise at this rate until the pres- 


the next 8 msec the gain of the amplifier remains 
constant, since contacts on the relay RC maintain 
the voltage on Cl and C2 at —5.25 v. This voltage 
is derived from the — 105-v regulated supply through 
the divider resistors Rl, R2, and R3. The voltage on 
these condensers supplies bias to the grids of the 
first two amplifier stages. When the relay RC opens, 
the condensers Cl and C2 begin to discharge through 
two parallel paths: one, through resistors R4 and 
R5 and two, through the 8-5 section of the diode 
and R7. The reduction in voltage on these condensers 
causes the gain to increase until it reaches a value 
such that the output signal from the amplifier is 
sufficient to produce a voltage across the 3-4 winding 
of T9 having a peak value equal to or greater than 


128 


BELL TELEPHONE LABORATORIES 157B AND 157C SYSTEMS 


the sum of the following two factors. One, the bias 
supplied to terminal 3 of T9 from the voltage divider 
consisting of R8, R9, and RIO and, two, the voltage 
existing on C3. When this voltage is exceeded, cur- 
rent will flow between terminals 3 and 4 of the diode 
causing terminal 3 to become negative with respect 
to ground by the amount by which the peak value of 
the output of T9 exceeds the first of the above bias 
factors. This voltage established across C3 and R7 
prevents discharge of the condenser C2 through the 
diode path. Since the voltage on C2 may now decay 
only through R4, the rate of gain increase in the am- 
plifier is reduced so that its maximum value now will 
be about 9 db per double time-interval. It must be 
remembered that the received signal during the por- 
tion of the receiving interval which has been con- 
sidered will normally consist primarily of surface re- 
verberation. The time constants are chosen so that 
the rate of increase in sensitivity of the amplifier will 
not be great enough to allow reverberation signal to 
pass the threshold stage. 

Now consider the manner in which the gain is con- 
trolled following the time when it has risen to the 
point where the reverberation signal has first caused 
conduction of the 3-4 section of the diode and re- 
duced the rate of gain increase. From this point on, 
until the end of the 1-sec listening period, the opera- 
tion will be as follows. The reverberation signal may 
be expected to decrease in level with time. If it de- 
creases at a rate faster than the rate at which the 
amplifier gain increases, because of the decay of the 
voltage of the condenser C2 through R4, the output 
at T9 will fall to a value insufficient to cause conduc- 
tion through the 3-4 section of the diode and the volt- 
age on C3 will decay through R7 until conduction 
through the 8-5 section of the diode is reestablished. 
When this happens, the gain will again increase at 
the rapid rate until the output level again rises to a 
point where the rapid gain increase is blocked. This 
type of control continues until the level of reverbera- 
tion signals falls to a value less than the signal caused 
by the self noise of the torpedo. The system is so de- 
signed that the amplifier gain cannot increase to a 
point where the expected self noise of the torpedo 
will produce a signal which can pass the threshold 
stage. 

So far, the performance of the gain-control circuit 
under the influence of reverberation and noise has 
been discussed. The action will now be considered in 
the presence of an echo signal. It has been observed 
that echo signals often consist of a smear of echoes 


representing signal returning from many points over 
a range of 50 yd or more. The value of signal neces- 
sary to cause conduction in the threshold stage is 
approximately 7.2 db greater than the value neces- 
sary to produce conduction in the 3-4 section of the 
gain-control diode. The latter is determined by the 
bias applied to terminal 3 of T9. The 7.2-db differen- 
tial is the value which field experience has indicated 
to be the minimum value necessary to exclude noise 
peaks. The lowest possible value is used in order that 
the minimum level of signal above the peak back- 
ground noise will be passed by the threshold circuit. 
It is obvious that when the signal level exceeds the 
noise and reverberation voltage, conduction will oc- 
cur in the 3-4 section of the diode and prevent fur- 
ther rapid increase in gain. If it were not for the 
feature which permits further slow increase of the 
gain, the maximum level of echo smear would need 
to exceed by at least 7.2 db the value of the rever- 
beration at which the gain increase was first inter- 
rupted in order to pass the threshold . In other words, 
if a smear echo is received at short range at the time 
when the reverberation is decaying rapidly, it is de- 
sirable to allow all of it to pass the threshold even 
though the distance attenuation causes the last por- 
tion to be less than 7.2 db greater than the reverbera- 
tion level at the beginning of the smear. The time 
constant of the combination of C3 and R7 is deter- 
mined primarily by the following consideration: The 
3-4 section of the diode in combination with C3 con- 
stitutes a peak rectifier, and the rate at which the 
voltage on C3 decays through R7 during the inter- 
vals between noise and reverberation peaks deter- 
mines the manner in which these peaks are inte- 
grated. The voltage on C2 decays only during the 
minimums of the voltage on C3. Therefore, the more 
rapidly C3 is allowed to decay the lower it will fall 
between noise peaks and the lower the final voltage 
on C2 will become. The choice of a time constant of 
15 msec for the C3 and R7 combination is one of the 
factors determining the choice of 7.2 db as the volt- 
age ratio. Any change in the time constant would 
necessitate a new choice for the voltage ratio. 

22.9 LIMITER CIRCUIT 

The limiter circuits following the threshold stages 
are conventional, single-stage, high-gain, class A am- 
plifiers. The limiting action is achieved simply by 
virtue of the fact that the signals applied to this 
stage are such that overload occurs when the voltage 


PHASE-SENSITIVE DETECTOR 


129 


output of the threshold stage is only 1 db greater 
than that necessary to pass the threshold. This is 
due to the fact that a 6SH7 tube overloads at ap- 
proximately 3 v of signal on the grid, and 1 db above 
the threshold of 52*. 5 v is a 6.5-v signal. A series re- 
sistance limits the grid current in this stage to small 
values and, therefore, prevents the grid from going 
appreciably positive. This enhances the limiting ac- 
tion. When the signal level is 2 db above that neces- 
sary to pass the threshold stage, the limiter stage 
will be completely limited. 


and 5-6 of FI and 5-6 of F2 are all phased in the 
same way. This means that the voltages in 3-4 and 
5-6 of FI will always be in phase and the voltages in 
5-6 of F2 will lead, be in phase, or lag those in FI, 
depending on whether the echo is received from the 
right, dead ahead, or left. Assume that an echo is 
received from the left, so that the right-channel sig- 
nal in 5-6 of F2 lags the left-channel signal in 3-4 
and 5-6 of FI by 45 degrees. These voltages are in- 
dicated in vector diagrams in Figures 13A, B, and 
C. The vector representing the voltage in 5-6 of F2 



22.10 PHASE-SENSITIVE DETECTOR 

It is the purpose of the phase-sensitive detector to 
combine the output of the two receiver channels in 
such a way that the difference in electrical phase be- 
tween them can be used to generate a usable signal. 
The essential elements of the detector are shown in 
Figure 12. F3 is a 90-degree phase-shift network and 
RV1 is a rectifier. The triode VI and the condenser 
C4 retain the information, which consists of a d-c 
voltage whose sign depends on the direction of in- 
cidence of the echo signal on the transducer and 
whose magnitude depends on the value of the angle 
between the torpedo axis and the direction of the 
incident echo signal. 

In the operation of the circuit, the windings 3-4 


will be delayed 90 degrees by the phase-shift net- 
work F3 so that the vector representing the voltage 
in 3-4 of F3 will be as indicated in Figure 13D. Re- 
ferring to Figure 12, it will be noted that the voltage 
A + C' is rectified by section 2-3 of R V 1 and applied 
across the resistance R28. Similarly the voltage 
C' — B is rectified and appears as a voltage across 
R29. These voltages will be added vectorially as in- 
dicated in Figures 13 E and F. When rectified, the 
relative magnitudes of voltage appearing across R28 
and R29 can be indicated as in Figure 13G. The 
plot of voltage at the top end of R28 referred to the 
lower end of R29 as a function of electrical phase 
angle is indicated in Figure 13H. 

When an echo is received, VI is made conducting 
so that the condenser C4 which was previously 


130 


BELL TELEPHONE LABORATORIES 157B AND 157C SYSTEMS 


charged to + 19 v is discharged to the value appear- 
ing across R28 and R29. The following three factors 
determine the choice of values of R28, R29, Cl, C2, 
and RV1. First, the storage condenser C4 reaches its 
final voltage through a series-resistant circuit con- 
sisting of the plate impedance of the tube V 1 and the 
load resistances R28 and R29. The sum of these com- 
ponents must be sufficiently small to allow C4 to 
reach its final voltage within 1 msec. This determines 
the maximum value for R28 and R29. Second, the 



A B C 


3*4 of FI 5-6 of FI 5-6 of F2 


A -B 



G 

Figure 13. Analysis of phase-sensitive detector opera- 
tion. (Refers to Figure 12.) 


condensers Cl and C2 act as filter condensers for 
removal of signal frequency from the echo pulses. 
The degree of filtering is a function of the ratio of 
the condenser impedance at signal frequency to the 
load resistance. The value of the capacitance is so 
chosen that 30 db of suppression of signal frequency 
voltage is obtained. Third, it is necessary that the 
shape of the rectified pulse shall be such that the 
final voltage is reached quickly. It is also necessary 
that following termination of the received signal, the 
pulse will fall off sufficiently slowly so that the grid 
of the tube V 1 can be biased to cutoff before the volt- 
age on C4 is appreciably modified because of the 


decay of the detector pulse. The rapid rise of the 
rectified pulse is determined by the size of the con- 
denser C4, the impedance of the varistor, and the 
impedance of the source supplying the detector cir- 
cuit. The source supplying the detector circuit is the 
plate circuits of the limiter stages whose impedance 
is modified by the filters FI, F2, and F3 and by the 
ratio of the output transformers FI and F2. The 
transformer ratio is determined by the output volt- 
age required from the detector, which is 30 v for a 
maximum phase difference of 90 degrees at the input. 
With this value determined and with the limiter 
tubes operating at saturation, the transformer ratio 
and hence the impedance are determined. The im- 
pedance of the varistor elements is chosen sufficiently 
low so that they are not limiting factors in deter- 
mining the impedance of this circuit. It is necessary 
to achieve a compromise between the quick charge 
of the condensers Cl and C2 and the quick-charging 
requirements of the condenser C4. 

22.11 PREFERENCE CIRCUIT 

The purpose of the preference circuit is to distin- 
guish among a number of echoes coming from a mov- 
ing target, and by proper presetting, to choose the 
one which comes from a point on the target nearest 
the bow. This is a means of ignoring the wake which 
is produced by the motion of the target through the 
water. It has been found desirable to incorporate a 
lead-angle feature in the preference circuit so that 
the course called for by the setting of the gyro cover 
plate leads the bearing of the echo by approximately 
4 degrees. The actual value of the lead angle can be 
varied from 0 to 8 degrees by proper circuit adjust- 
ments. The circuit elements involved in this feature 
are shown schematically in Figure 14. The ability 
to choose the rightmost or leftmost echo is achieved 
by means of the phase-sensitive detector VI, the 
relay LR2, and the condenser C4 which stores the 
output of the phase-sensitive detector. In operation, 
the condenser C4 is charged at the beginning of each 
listening period to + 19 v. When an echo passes the 
threshold circuits this condenser will be discharged to 
a voltage equal to the output voltage of the phase- 
sensitive detector. If succeeding echoes are received, 
the value of voltage assumed by the condenser C4 
will be equal to the new value of the phase-sensitive 
detector output voltage providing this voltage is 
negative with respect to the previous signal. The 
fact that the tube VI conducts in only one direction 


PREFERENCE CIRCUIT 


131 


prevents the potential of the condenser C4 from being 
changed in a positive direction. The relay LR2 in- 
dicated in Figure 14 is used to connect the phase- 
sensitive detector to the condenser C4 so that either 
port or starboard echoes will make the potential of 
the top of the condenser more negative. The system 
is preset for preferred-side steering, by setting up 
the relay LR2 so that echoes from the preferred side 
make the top of the condenser C4 most negative. 


voltage equal to that applied to the cathode circuit 
by the output of the phase-sensitive detector. At the 
termination of the echo signal VI is again cut off so 
that further discharge of the condenser C4 is pre- 
vented. A later signal, less positive than the preced- 
ing one, will repeat the sequence and permit the volt- 
age on C4 to fall to a still lower value. However, a 
succeeding signal more positive than the preceding 
one will not alter the voltage of C4 since, in this case, 



The operation of the system is as follows. The 
condenser C4 is charged by means of a contact on 
the relay ST to +19 v with respect to its terminal 
which goes to the junction of R6 and R7. It is pre- 
vented from discharging in the absence of echo sig- 
nals by VI which is biased to plate-current cutoff by 
a suitable tap on the voltage divider consisting of 
the resistors R4, R5, R6, R7, and R8 and the follow- 
up potentiometer in the translator. The output of 
the phase-sensitive detector, which is developed 
across the resistors R28 and R29, is connected in the 
desired sense through the contacts of the relay LR2 
between the cathode of VI and one side of C4. When 
an echo signal passes the threshold circuit, and after 
a suitable delay which is introduced in the trigger 
circuit, the grid of VI is driven positive. The con- 
denser C4 is then free to discharge through VI to a 


the cathode of VI will be positive relative to its plate 
and the tube will not conduct in that direction. The 
manner in which the grid of VI is driven positive 
will be explained under Section 22.12. 

The value of the condenser C4 is determined as a 
compromise between two factors. First, a discharge 
time constant of approximately 0.2 msec is required. 
This determines the maximum size of C4. Second, it is 
necessary for the circuit leakage resistance to be very 
high. It is essential that the voltage due to charge 
stored on C4 shall be capable of remaining unaltered 
by circuit leakage for a period of 1 sec. This deter- 
mines the minimum size for C4 and also the minimum 
possible circuit leakage resistance including the leak- 
age resistance of C4 itself. The circuit leakage is mini- 
mized by the use of a guard circuit which holds to a 
minimum the voltage drop across the leakage path. 



132 


BELL TELEPHONE LABORATORIES 157B AND 157C SYSTEMS 


22.12 TRIGGER CIRCUIT 

The trigger circuit, also indicated in Figure 14, 
performs two functions following the receipt of an 
echo signal which passes the threshold. First, it makes 
VI conducting by removing its bias after the out- 
put of the phase-sensitive detector has reached equi- 
librium and reestablishes the bias at the proper time 
in relation to the output pulse from the phase-sensi- 
tive detector. Second, it fires the gas tube V2. 

The first function of the circuit is achieved in the 
following manner: a portion of the output of the 
limiter in the starboard channel is obtained from the 


ducting until the output of the phase-sensitive de- 
tector has had opportunity to reach its equilibrium 
value. It should be noted that the grid-to-cathode 
voltage of VI is actually a composite of the trigger- 
circuit voltage and the phase-sensitive detector volt- 
age. This is somewhat undesirable since an output 
of the phase-sensitive detector of such a polarity as 
to cause the cathode of VI to become negative with 
respect to the junction of R6 and R7 will, in effect, 
reduce the bias on the tube and, therefore, tend to 
cause it to conduct in the absence of a trigger signal. 
However, in practice, the grid-to-cathode voltage of 
VI from this source is also delayed by the combina- 





3-4 winding of the filter transformer F2. This is rec- 
tified in the voltage-doubler rectifier circuit consist- 
ing of the 1-3 arm of RV4 and the condensers Cl and 
C2 developing a voltage across the load resistor R2. 
The rate of rise of voltage across R2, following re- 
ceipt of an echo signal, is somewhat faster than the 
rate of rise of the phase-sensitive detector output 
voltage across R28 and R29, because of the relative 
magnitudes of the condensers Cl and C2 and the 
source impedance. This voltage is applied to the grid 
of VI through the resistor R3. The resistor R3 and 
condenser C3 serve to delay the rise of voltage on 
the grid of VI so that the tube does not become con- 


tion of R3 and C3 which prevents the tube from be- 
coming conducting in the absence of trigger circuit 
signal. At the termination of an echo signal, the bias 
on VI is rapidly restored to a negative value, since 
the condenser C3 discharges through the 4-6 arm of 
RV4 which shunts the resistor R3 for current flow in 
this direction. In this manner VI is cut off before the 
phase-sensitive detector output voltage has fallen 
appreciably from its equilibrium value. 

The second function of the trigger circuit, to fire 
V2, in turn operates the TG relay and so initiates a 
train of relay operations essential in the control of 
the torpedo. It has been found desirable to delay the 



SERVO SYSTEM 


133 


firing of the tube V2 beyond the beginning of an echo 
signal by about 1.5 msec. This delay, introduced to 
make the circuit less sensitive to peaks of self noise 
of the torpedo, is accomplished through the action 
of the resistor R9 which is shunted by the resistors 
RIO and Rll and the condenser C5. The choice of a 
1.5-msec delay is dependent on the choice of 7.2 db 
as the amount a signal must exceed the noise voltage 
in order to pass the threshold stage. The delay in the 
firing of the tube V2 is also associated with the delay 
in the reduction of the bias on the tube VI. These 
delays should be of approximately the same magni- 
tude, since it obviously is undesirable for V2 to fire 
before VI becomes conducting and modifies the volt- 
age on the condenser C4. 

22.13 SERVO SYSTEM 

The electromechanical system, following the C4 
condenser shown in Figures 13 and 14, translates 
electrical information into mechanical rotation. The 
essential parts of this system indicated in Figure 15 
are a d-c amplifier, two control gas tubes, a trans- 
lator, and a follow-up potentiometer. 

The voltage on Cll remains there for about the 
time required for maximum rotation of the gyro 
cover plate. The high side of Cll is connected to the 
grid of VI, a coupling stage, which was selected for 
its very high input resistance to prevent the charge 
on Cll from leaking off. The low side of Cll is con- 
nected to a voltage divider between +150 and — 105 
v which also contains the follow-up potentiometer in 
the translator. When the follow-up potentiometer is 
centered, the voltage at this point is zero. 

The two halves of V2 and the thyratrons V3 and 
V4 are connected as two parallel d-c amplifiers, one 
of which operates on a positive signal on grid ter- 
minal 1 of V2, and the other operates on a negative 
signal on grid terminal 1 of V2. The plates of V3 and 
V4 are connected through relay contacts to the right 
and left rudder clutches in the translator in series 
with 150-v, 400-c alternating current. The potenti- 
ometer P3 sets the normal grid bias on these tubes 
so that for 0 v on Cll and the follow-up potenti- 
ometer in mid-position, they are extinguished. 

With the system set for right preference, an echo 
from the left produces a positive voltage on Cll. 
This breaks down V3 in the following way. Referring 
all potentials to the junction of R1 and R2 which is 
at — 105 v, when the positive voltage on Cll makes 


the grid of VI positive the current in the cathode cir- 
cuit increases, whereupon the grid 1 of V2 becomes 
more positive because of the drop in R3. This in- 
creases the cathode current in the 1-2-3 half of V2. 
Because of the drop in R2 the cathode of the 4-5-6 
section becomes more positive, which reduces the 
drop in R9 and makes the grid of V3 more positive, 
causing it to fire. When V3 breaks down, it energizes 
the left gyro clutch which is located in the translator. 
A right echo produces a negative voltage on Cll 
which breaks down V4 in a similar fashion. 



Figure 16. Translator with cover removed. 


The construction of the translator and its coupling 
to the generator and power supply are shown in 
Figures 16 and 17. 

The main drive for the translator is obtained from 
the driveshaft via the generator. Left and right gyro 
cover plate corrections are obtained by means of a 
pair of jaw clutches operated by a differential sole- 
noid. One-half of each clutch is free-running on the 
shaft and is driven continuously and in the opposite 
direction from the corresponding half of the other 


134 


BELL TELEPHONE LABORATORIES 157B AND 157C SYSTEMS 


clutch. The other two halves are in one piece and 
are splined to the shaft. 

The operation of one of the control tubes V3 or 
V4 energizes the solenoid in one direction, engaging 
one of the jaw clutches and causing the translator 
shaft to rotate in the desired direction. Through gear 
trains, this rotation is transferred to the gyro cover 
plate. The shaft driving the gyro cover plate is on 
the side of the immersion gear which is shown in 


of correction which can be applied to the cover plate 
by one operation. Another cam makes a contact 
when the shaft is in a position corresponding to the 
centering of the potentiometer. The heart-shaped 
cam, driven by a spring-loaded arm, returns the shaft 
to its center position when the potentiometer clutch 
is de-energized. This signifies that the translator is 
ready for more information. The other two cams are 
not used. 



Figure 18. The gyro cover plate switch is also shown 
in this picture. Its principal function is to limit the 
maximum excursion of the cover plate. It also con- 
tains the preference preset switch which is operated 
by turning the cover plate manually through the 
side-setting gear. 

The translator shaft also drives, through a mag- 
netic clutch, the cam shaft on which the follow-up 
potentiometer is mounted. Two cams on this shaft 
are used to operate switches that limit the amount 


When an echo is received, the operation of a relay 
energizes the follow-up potentiometer clutch and, as 
the cover plate drive rotates, the potentiometer fol- 
lows along with it. Taking as an example an echo 
coming from the left with the circuit set for left 
preference, V3 in Figure 15 becomes conducting and 
energizes the left gyro clutch which causes the cover 
plate drive to turn in that direction. When the 
follow-up potentiometer goes off-center in that di- 
rection it introduces a voltage between the low side 



RELAYS AND OVERALL SYSTEM OPERATION 


135 


of Cll and the cathode of VI. When this becomes 
equal to the positive voltage on the high side the net 
effect is the same as zero voltage on Cll, and V3 is 
extinguished . In this way the cover plate drive satis- 
fies the demand of ‘the condenser. 

The same procedure would occur if an echo came 
from the right except that the high side of Cll would 
become negative, V4 would break down, and the 
right gyro clutch would engage and turn the poten- 
tiometer so as to put a positive voltage on the low 
side of Cll. This would balance the negative voltage 
on the high side and would extinguish V4. 



Figure 18. Modified immersion gear showing method 
of driving gyro cover plate from translator. 


The whole servo system may be considered as an 
electromechanical amplifier employing negative feed- 
back from the follow-up potentiometer to the grids 
of V3 and V4. The phase relations, however, are not 
adjusted to the point where sustained oscillations 
are suppressed. Because of the delay between the 
signal which operates the clutches and the turning 
of the potentiometer which opposes the signal, there 
is a tendency to oscillate or hunt at a frequency of 
about 2 c. This hunting can be observed in the right 
and left clutch lights on the recorder traces when no 
echoes are being received. 

22.14 RELAYS AND OVERALL SYSTEM 
OPERATION 

The naming of the relays has been selected as an 
aid in understanding their functions. RC means re- 


verberation control relay, ST means starting relay, 
TG means trigger relay, and PC means follow-up po- 
tentiometer centering relay, since it operates follow- 
ing the closure of the contact which is closed only 
when the follow-up potentiometer is centered. 

These relays do not constitute all of the relay func- 
tions employed in the operation of the panel; how- 
ever, in combination they accomplish all of the func- 
tions associated with the utilization of echoes when 
once they are received. The functions which they 
accomplish are in general not accomplished by one 
relay alone; hence, three relays will be discussed as 
a group and not separately. The functions which 
this group of relays serve to accomplish are eight in 
number : 

First, to charge C4, Figures 14 and 15, to +19 v 
at the time the signal is transmitted. 

Second, to short-circuit Cll, Figures 14 and 15, 
thus eliminating any accumulated charge during 
periods in which no echoes are being received. 

Third, to release the follow-up potentiometer 
clutch and hence permit the follow-up potentiometer 
to center, following the receipt of an echo signal. 

Fourth, to transfer the voltage from C4 to Cll, 
Figures 14 and 15, following receipt of an echo signal 
and closure of the follow-up potentiometer contact. 

Fifth, to apply cutoff bias to the clutch-control 
tubes V3 and V4, Figure 15, during the period when 
the potentiometer clutch is de-energized, thus pre- 
venting the clutches from being operated while the 
potentiometer clutch is de-energized. 

Sixth, to disconnect C4 from Cll at the end of a 
receiving interval during which the voltage was 
transferred from C4 to Cll. This allows the voltage 
to remain on Cll during the succeeding receiving 
interval. 

Seventh, to remove the plate-supply voltage for 
V2, Figure 14, which flows through the TG relay 
winding at the end of each receiving interval. This 
serves to extinguish V2 at the end of each receiving 
interval provided it has been fired by an echo re- 
ceived in that interval. 

Eighth, to delay the transmission of the succeeding 
pulse until the follow-up potentiometer contact is 
centered, provided an echo has fired V2, Figure 14, 
prior to the end of the receiving interval. 

The manner in which the above functions are used 
to provide operation of the preference circuit and to 
pass the information to the d-c amplifier will now be 
described with reference to a set of typical operating 
conditions. 


136 


BELL TELEPHONE LABORATORIES 157B AND 157C SYSTEMS 


In general, all normal functions of the torpedo 
may be subdivided into three groups of typical con- 
ditions. First, the system is operating but no echoes 
are being received; second, the echoes are being re- 
ceived regularly in each pulsing interval; and third, 
the performance of the system following the receipt 
of an echo which in turn is followed by a pulsing in- 
terval in which no echo is received. 

For the consideration of the first condition in 
which the system is operating but no echoes are be- 
ing received, the action of the relays is as follows: 
Relays TG and PC remain released at all times, C4 
and Cll, Figures 14 and 15, are not connected to- 
gether, and the follow-up clutch is continuously en- 
gaged. Gas tubes V3 and V4, Figure 15, are enabled 
and the follow-up potentiometer will be hunting 
about the center position as discussed under the d-c 
amplifier. The ST relay operates once each second 
and performs the following functions. It connects C4 
to +19 v, short-circuits Cll, Figures 14 and 15, re- 
moving any leakage charge that might tend to build 
up at this point, operates the RC relay, which in turn 
charges the TVG condensers Cl and C2, Figure 11, 
and triggers the 3-msec multivibrator. It will be re- 
membered that the latter controls the transmitter. 
All of these functions repeat once each second. 

For the second condition, in which echoes are being 
received in each pulsing interval, assuming that no 
echoes have previously been received and the dis- 
cussion is begun just prior to the transmission of the 
pulse which is to produce the first received echo, the 
functions are as follows: The operation of the ST 
relay charges C4 to + 19 v, shorts Cll to remove any 
accumulated charge, and operates the RC relay to 
charge the TVG condensers and trip the 3-msec 
transmitter pulse. The ST relay releases approxi- 
mately 31 msec after the transmitted pulse. This 
enables the TG relay, opens the charging connection 
to C4, removes the short from Cll, and removes 
power from the reverberation control relay. C4 and 
Cll are disconnected from each other as previously 
noted. (Figures 14 and 15 should be referred to in 
discussion involving C4 and Cll.) Approximately 40 
msec after the transmitted pulse, the RC relay re- 
leases, its power having been removed by the release 
of the ST relay. This allows discharge of the TVG 
condensers to begin. The system is now in condition 
to receive an echo which will be assumed to arrive at 
some time between 40 msec and 991 msec after the 
transmitted pulse. When this echo arrives and passes 
the threshold, voltage from RV4 in Figure 14 fires 


the trigger tube V2 and operates the TG relay. Si- 
multaneously, the voltage on C4 is modified by the 
action of the phase-sensitive detector and the dis- 
charge tube VI. Operation of the TG relay performs 
the following functions: Cll is shorted, V3 and V4 
in Figure 15 are disabled, since the common point of 
R18 and R19 which was previously grounded through 
a back contact of the TG relay is now free from 
ground, so that +300 v is supplied through the 
follow-up clutch, and R19 biases the cathodes of V3 
and V4 to approximately +140 v. This same opera- 
tion also disengages the follow-up clutch, because of 
the high resistance of R17 and R18 inserted in the 
clutch circuit. Additionally, the TG relay disables 
the ST relay to prevent the possibility of its opera- 
tion until the next step to be mentioned has had 
time to be completed. The disengagement of the 
follow-up clutch allows the follow-up potentiometer 
to be centered if it is not already in that position. 
Centering of the follow-up potentiometer closes a 
contact energizing the PC relay. The operation of 
the PC relay performs the following functions. The 
ST relay is re-enabled; the short is removed from 
Cll, and C4 and Cll are paralleled (Figures 14 and 
15). V3 and V4, Figure 15, are re-enabled and the 
follow-up clutch re-engaged by re-establishment of 
ground on the junction of R18 and R19. Paralleling 
of C4 and Cll places a control voltage on Cll, and 
the d-c amplifier and translator act to translate this 
information to the gyro cover plate. No further ac- 
tion occurs until the next normal operation of the ST 
relay, whereupon the function is as follows, and dif- 
fers in important respects from the operation when 
no echoes were being received: The TG relay is de- 
energized; the PC relay, however, is held operated 
through its own 1-2B contacts and the 5-6B con- 
tacts of the ST relay. C4 and Cll are discon- 
nected from each other and C4 is recharged to +19 
v. Cll, however, is not shorted as it was when no 
echo was received. This difference is achieved by 
virtue of the fact that the PC relay is now in the 
operated position. The RC relay is operated, the TVG 
condenser is charged, and the pulse transmitted as in 
the previous sequence. Upon release of the ST relay 
all the functions are the same as in the preceding se- 
quence except that the PC relay is also released. 
Operation from this point on will be identical to the 
above operation as long as echoes continue to be re- 
ceived regularly in each pulsing interval. 

For the third and final condition, in which follow- 
ing the receipt of an echo which in turn is followed 


RELAYS AND OVERALL SYSTEM OPERATION 


137 


by a pulsing interval in which no echo is received, 
the functions are as follows. Begin the consideration 
with the operation of the ST relay which is closed 
during the time a pulse is transmitted for which no 
echo is received. Iii this case all of the operations of 
the ST relay are the same as were noted for the case 
in which echoes were being received, and, following 
its release, the voltage on Cll remains unaltered 
from that value which was placed on it by the echo 
received in the previous interval. The translator has 
presumably acted to translate this voltage to a cor- 
responding angle on the gyro cover. Since no echo 
is received during this receiving interval, the TG 
relay does not operate. Hence, upon the next opera- 
tion of the ST relay, the following operations occur: 
Since the TG and PC relays have not been operated, 
Cll is shorted, but the follow-up clutch is not re- 


leased. In order for the follow-up clutch to be re- 
leased the relay TG must be operated and the PC 
relay not operated. The rotation of the follow-up po- 
tentiometer due to the preceding voltage on Cll 
now produces an unbalanced condition at the input 
to the d-c amplifier and action of the amplifier and 
translator is initiated to restore a condition of bal- 
ance. This requires a rotation of the follow-up poten- 
tiometer back to zero to balance zero voltage on Cll. 
Hence the rotation previously translated to the gyro 
cover is now removed. All other operations of the 
ST relay are similar to those which occurred in the 
first condition described where no echoes were re- 
ceived . 21 

a See references 48-57 for additional material on topics in 
this chapter. 


Chapter 23 


GEIER TORPEDO CONTROL SYSTEM 


23.1 INTRODUCTION 

A t the end of 1942 the Atlas Werke, Munich 
undertook the development, under the code 
name Boje , an acoustic homing control system for 
torpedoes employing echoes received from the target 
ship rather than ship’s noise. Tests conducted at 
Obertello, Italy, showed that, for distances of a few 
hundred meters, the echo-to-reverberation ratio is 
independent of range and depends only on the direc- 
tivity index of the transducers. It was therefore de- 
cided that transducers having a high directivity index 
should be employed. In order to achieve this high 
index it was necessary to use a high frequency for 
the signal generator. There was a demand for a quick 
solution to the problem so, since Atlas Werke pos- 
sessed only one standard nickel lamination trans- 
ducer design operating at a frequency greater than 
25 kc, namely a 77.5-kc unit, rather than tool up to 
produce nonstandard laminations, it was decided to 
use 77.5 kc. It was thought simpler to use separate 
projectors and hydrophones than to switch the 
output-input circuits to a single pair of transducers. 
Hence the space allotted to each transducer was se- 
verely limited. 

By the end of 1943 the first laboratory models had 
been completed and tested against stationary and 
moving targets at Gdynia. After a few changes re- 
sulting from these trials, a preproduction design was 
frozen and designated as Geier 1. The first trials of 
this model were held in March 1944. Subsequently 
about 120 Geier 1 units were produced and most of 
them were installed in torpedoes. A few of these units 
were nonstandard, various modifications of the steer- 
ing mechanism being tried. By the end of 1944 sev- 
eral hundred experimental torpedo shots had been 
fired at Gdynia. 

In the meantime, it was decided to incorporate all 
the changes shown to be desirable by the Geier 1 
trials in a new version, Geier 2. It was intended that 
Geier 2 be the service torpedo; Geier 1 was to have 
been the guinea pig. The development of Geier 2 
was shared between Atlas Werke, Munich and 
Minerva Radio, Vienna. The first Geier 2 sea trials 
were held in the fall of 1944 and, by the time ac- 


tivities were halted, about 20 experimental shots of 
Geier 2 had been fired. Shortly before the surrender 
of Germany, about 100 of the preproduction Geier 1 
torpedoes were transferred from experimental use to 
operational status. However, it appears that none of 
these torpedoes were fired in action as they were later 
located in a depot. 

Although there are these two principal versions of 
Geier, and each of them exists in a number of slightly 
different designs since the final version had not been 
decided upon at the close of the war, all the designs 
operate on the same basic principle. The torpedo is 
equipped with two magnetostriction projectors and 
two hydrophones, all four units being of the same 
design. These transducers are mounted in the tor- 
pedo nose, in the manner shown schematically in 
Figure 1. The projectors are excited simultaneously 



from the same source. Each hydrophone is connected 
to its own amplifier channel. There is no comparison 
of the intensities of the echoes received in the two 
channels; instead, each channel possesses a definite 
threshold, above the reverberation or background 
noise, which varies as a function of time after trans- 
mission and which the signal must exceed in order 
to exercise control. The steering method, which is of 
the cut-on cutoff type, exists in two variations, known 


138 


GEIER 1 


139 


as symmetrical steering and preferred-side steering. 
When echoes are received on only one side, the two 
systems behave in the same way: the torpedo steers 
toward the echo side for a definite time and then re- 
turns to gyro control until another echo is received. 
In case echoes are received from both sides, the sym- 
metrical steering system is governed by the first echo 
to arrive within a given ping interval. The preferred- 
side steering system is biased either to port or star- 
board just before the torpedo is fired, with the result 
that it always steers to the preferred side if an echo 
is received on that side, regardless of whether or not 
an echo from the other side has been received in the 


and the latter probably the F5b. In both designs the 
transducers and electronic components are located 
in the torpedo nose. In the submarine torpedo the 
gyro assembly is equipped with solenoids in a manner 
similar to, if not identical with, the corresponding 
( Specht ) system in T5. These solenoids are operated 
by relays in the forward electronic assembly. Power 
is obtained from a generator on the main propulsion 
shaft. 

23.2 GEIER 1 

The application of Geier 1 to the submarine tor- 
pedo G7e had proceeded much further than the air- 



Figure 2. Block diagram of Geier 1. 


same ping interval. The following specifications are 
common to all designs: 

Frequency 77.5 kc 

Pinging power electric input to each 

projector 100 watts 

Ping interval 0.33 sec 

Maximum reliable acoustic control 

range 200 meters 

The Geier control was applied to both submarine 
and aircraft torpedoes, the former being the G7e 


craft version. In the following paragraphs the sub- 
marine application is discussed in detail, and later 
the modifications for the aircraft version are noted. 

The principal features of Geier 1 may be under- 
stood by reference to Figure 2. The projectors are 
excited by discharging a condenser through the wind- 
ing. The condenser is connected to the projectors for 
a short interval by a contactor segment mounted on 
the rotating shaft of the mechanical time base. This 
shaft is driven by a small electric motor. After the 
condenser has discharged, it is disconnected from the 


140 


GEIER TORPEDO CONTROL SYSTEM 


projectors and allowed to charge again from a high- 
voltage d-c power supply. Figure 3 shows the se- 
quence of events during one ping interval. 


223 


77Z777ZZZZ 2L 

CAMS 384 I 


CAMS 

cams 


CAM 2 


| 'W///////////ZZL 


I 

I 

-20 


TRANSMITTER 


AMPLIFIER BLANKING 


I 

I ! CHARGE RELAY CONDENSER 

! I 

V////////////////////////////A TVG 


TIME IN MILLISECONDS 

Figure 3. Time base of cams. 


Each hydrophone is connected to its own ampli- 
fier. The gain of the amplifier is suppressed during 
the transmission of the ping; during the remainder 
of the ping interval the gain is controlled by a time- 


torpedo steering engine. This assembly, which em- 
bodies the symmetrical-steering method, has two 
output channels. One of these operates a solenoid 
which disengages the gyroscope from the steering 
engine; the second channel operates solenoids which 
control the steering engine to give full helm to port 
or starboard. For example, if the torpedo receives an 
echo signal in the starboard channel, the gyroscope 
is disengaged and the helm put hard to starboard. 
The disengage relay is held in for 0.6 sec. If by that 
time no more echoes have been received in either 
channel, the disengage relay drops out and the tor- 
pedo returns to gyro control. If, after the first echo 
is received, another is received in the same channel 
in the next ping interval, the disengage relay con- 
tinues to hold in, and the gyro remains disengaged 
for 0.6 sec after receipt of the last echo. When the 
torpedo comes close to the target or its wake, it is 
probable that echoes will be received in both chan- 
nels during the same listening interval. The relay 



3 A SC CAM 3 

Figure 4. Amplifier showing TVG and A VC systems. 


varied gain [TVG] circuit and a relatively slow- 
acting automatic volume control [AVC] circuit. The 
TVG circuit serves to reduce response to reverbera- 
tion to a nearly constant level, while the AVC circuit 
corrects for differences in absolute level, such as those 
caused by torpedo roll and changing sea state. 

The relay assembly is the connecting link between 
the electric signal output of the amplifiers and the 


assembly includes a pre-emptive feature which per- 
mits it to ignore all signals other than the first which 
arrive within a given listening interval. Thus the 
torpedo tends to steer towards that part of the acous- 
tic target to which it is closest. 

Each transducer, resonant at the nominal fre- 
quency of 77.5 kc, consists of a rectangular block of 
nickel laminations slotted to provide space for the 


GEIER 1 


141 


winding. The radiating face is 4.4 X 8.5 cm. The im- 
pedance at resonance is approximately 10 ohms. The 
four transducers, mounted as shown in Figure 1, are 
enclosed in a nose cap, or dome, made of a thermo- 
plastic material. This dome is similar to or identical 
with the dome used in the round-nosed T5 listening 
torpedo. The space behind the dome is filled with 
ethylene glycol. The two-way transmission loss 
through this coupling system is stated to be 10 db. 

The transmitter in Geier 1 consists simply of a 
1-^tf condenser which is charged during the listening 


mission. The condenser C4 is charged to a positive 
potential determined by the circuits consisting of PI, 
R3, R4, R5, R6, and R8. At the end of the 33 msec, 
cam 2 breaks the circuit between Cl and ground and 
Cl as well as the grid of VI begin to approach the 
potential of C4 which is at the potential of the cath- 
ode of VI. This causes the gain in VI to increase and 
the rate of increase in gain is controlled largely by 
Cl and R2. The setting of PI determines the gain at 
the time of start of the gain increase. 

The following two stages, indicated in Figure 4 in 


TO PORT jo STARBOARD 

STEERING STEERING 



Figure 5. Relay system. 


interval by means of a transformer and rectifier op- 
erating from a 36-c, 17-v a-c generator. The condenser 
is discharged through the tuned projectors by means 
of one of the cam switches in the time base. 

The two receivers for the starboard and port chan- 
nels are independent and identical. Figure 4 is a 
schematic of one of them. It consists of a 3-stage 
radio-frequency amplifier followed by a single stage 
pulse amplifier for operating the relays. The first 
stage VI in Figure 4 is controlled by TVG. Cam 2 in 
the time base connects the grid of VI and the con- 
denser Cl to ground until 33 msec following trans- 


block-diagram form, are similar to VI except that 
their grids are simply returned to the AVC line 
through 1 -megohm resistors. These two stages are 
under AVC but not under TVG control. 

When a signal appears at the junction of C7 and 
C8 it is necessary for it to have a peak value greater 
than 12 v before rectification can take place in the 
diode and the rectifier RV2. When rectification does 
take place, point A becomes more negative and point 
B becomes more positive and C9 acquires a further 
negative charge from point A via the resistor RIO. 
This applies a greater negative bias to the grids of the 


142 


GEIER TORPEDO CONTROL SYSTEM 


second and third stages of the amplifier which de- 
creases their gain. 

If the increase in signal level takes place slowly 
enough there will be very little change in the poten- 
tial of the grid of the pulse amplifier V3 with the re- 
sult that it will not become conducting. However, 
because of the long time constant imposed on the 
rate of development of negative potential by the con- 
denser C9, a rapid increase in signal level will result 
in the grid of V3 becoming more positive. The exact 
conditions for firing V3 can be controlled by adjust- 
ing the potentiometer R12. 

It will be noted that the plate of V3 is connected 
by way of the gas tube V4 shown in Figure 4 and 
the coils of relays 1, 2, and 3 in the control circuit 
shown in Figure 5 to the condenser Cl. The con- 
denser Cl is charged to +200 v just prior to the time 
the transmitter sends out its pulse. The voltage on 
this condenser is divided between the gas tube V4 
and the plate of V3 in Figure 4. When a positive 
pulse arrives on the grid of V3 it causes V4 to break 
down and the condenser Cl in Figure 5 discharges 
through the relay coils and the plate circuit of V3. 
Once the discharge has started it will continue, using 
the rectifier RV3 as a return path even if V3 becomes 
cut off again. 

The action of the relay assembly is as follows. 
Relays 1, 3, and 4 are polarized differential relays, 
whose armatures stay in the position to which they 
were last moved by a current impulse. Relay 2 is a 
spring-controlled relay which opens when no current 
is passing. Relay 4 is the gyro-disengage relay. When 
current passes through winding 9-10 the armature is 
in the Z position, which means the gyroscope is cou- 
pled to the steering engine. When current passes 
through winding 1-5, the armature moves to the T 
position, energizing the gyro-disengage solenoid. 
Relay 1 is the steering relay; when current has last 
passed through winding 9-10, the armature is in the 
Z position, energizing the port solenoid, and contrari- 
wise when current has last passed through 1-5. Re- 
lay 3 is a relay whose contacts are in series with the 
power supply energizing all the solenoids. When 
current is passing through winding 9-10, the elec- 
trical steering is disabled. This circuit feature pro- 
vides the delay in initiating acoustic control re- 
quired for safety purposes. The delay is provided by 
the thermal delay switch SW1, which opens after a 
definite time. Relay 3 is operated to the other, or 
closed, position by the first signal impulse which ar- 
rives from either channel after SW1 has opened, and 


remains in this position thereafter. Relay 2 is an 
intermediate relay whose armature is in the Z posi- 
tion when not operated by current. 

Suppose now that the initial safety delay has 
elapsed, so that SW1 is open. Terminal 13 of the 
assembly is connected to the positive side of the 
200-v supply, while terminal 4 is connected to the 
negative side of this supply. Relay 2 is in the Z posi- 
tion, shorting the 1-5 winding of relay 4. C2 charges 
through R3 until the gas tube VI breaks down, send- 
ing a pulse of current through the 9-10 winding of 
relay 4. The period of this simple relaxation oscilla- 
tor is 0.6 sec. Relay 4 stays in the Z position and the 
torpedo remains under gyro control. During the ping 
transmission, terminal 14 of the control circuit ( Veil - 
chen) is briefly connected to the 200-v supply by the 
last contactor on the time base (terminals 9 and 10). 
Thus Cl is charged to this potential. For the re- 
mainder of the ping interval Cl is disconnected from 
the high-voltage supply. If a signal, having sufficient 
amplitude to fire the glow tube V4 in the amplifier 
shown in Figure 4, arrives in the starboard channel, 
Cl discharges through the protective resistor R6 and 
windings 1-5 of relays 3, 2, and 1. Relay 3 is moved to 
the T or closed position and remains there for the 
rest of the run. Relay 2 moves momentarily to the T 
position and returns to Z after Cl is discharged; 
relay 1 moves to the T position, if not already there, 
energizing the starboard-steering solenoid. The mo- 
mentary operation of relay 2 has two results. First, 
C2 is discharged, so that winding 9-10 of relay 4 will 
not receive another pulse until 0.6 sec has elapsed. 
Second, the short is removed from 1-5 of relay 4, so 
that a pulse of current passes, moving relay 4 to the 
T position. Thus the gyro disengage solenoid is op- 
erated, and the steering engine passes under the con- 
trol of the starboard solenoid. If after 0.6 sec has 
elapsed no more echoes have been received, C2 will 
again discharge through the gas tube V 1 and winding 
9-10 of relay 4, thus restoring the torpedo to gyro 
control. On the other hand, if an echo is received in 
the next listening interval, before the 0.6 sec have 
elapsed, relay 2 is again momentarily operated to the 
T position, again discharging C2 and ensuring a 
further delay of 0.6 sec before the torpedo is re- 
turned to gyro control. 

The purpose of supplying the firing voltage for the 
glow tube in the amplifier from Cl of the control 
circuit is to provide the pre-emptive feature men- 
tioned above, whereby the circuit ignores all but the 
first echo in a given listening interval. Clearly, once 


GEIER 2 


143 


this condenser has been discharged as a result of the 
first echo, none of the relays may be operated again 
until Cl is recharged at the beginning of the next 
ping interval. 

No records of*the many experimental trial shots 
were available at Atlas Werke, but some general 
conclusions regarding the performance of Geier 1 
were stated by Atlas personnel who had attended 
the trials. On bow shots the performance was good, 
but for shots made aft of the beam of the target, in 
many cases the torpedo attempted to home on the 
wake, crossing the latter at right angles. After doing 
so, it would return to gyro control and proceed away 
from the target. This result is in qualitative agree- 
ment with an analysis of the symmetrical type of 
steering, which is discussed in more detail in Section 
23.4. It was stated that the gear was responsive to 
ship or decoy noise; in particular, the torpedo was 
said to be able to home on ship’s noise at a range of 
500 m provided the noise is modulated. If the noise 
is steady in level, then the only fluctuation appearing 
at the grid of the fourth stage of the amplifier is that 
caused by the increase of gain resulting from the ac- 
tion of the TVG circuit. Preliminary tests show that 
if a steady signal of single frequency is applied to the 
input circuit, this type of fluctuation is sufficiently 
slow so as to be very effectively removed by the 
action of the A VC circuit. On the other hand, if the 
ship’s noise fluctuates rapidly, the peaks will not be 
suppressed by the A VC circuit and will appear like 
echo signals. Then, the only condition necessary for 
operation of the control circuit by such noise peaks 
is that they exceed the reverberation, or the steady 
average noise background, by the amount deter- 
mined by the setting of potentiometer R12 in Fig- 
ure 4. 

Some difficulty was experienced with increased re- 
verberation caused by torpedo pitch and roll. Since 
reverberation rather than self noise is the limiting 
factor in determining whether or not useful echoes 
are received, this problem was given serious con- 
sideration. A transducer assembly stabilized both in 
roll and pitch was developed. 

The electronic circuits for the aircraft version are 
the same as those used in the submarine torpedo; 
but since the plastic nose used in the latter is too 
weak to withstand water-entry shock, a different de- 
sign had to be produced for the aircraft torpedo. It 
was found very difficult to reconcile the requirements 
of mechanical strength and acoustic transparency. 
The first attempt consisted of mounting the trans- 


ducers directly in the steel nose cap of the conven- 
tional torpedo, so that the radiating surfaces were in 
direct contact with the water. This arrangement was 
satisfactory from the strength viewpoint and of 
course provided the best possible coupling between 
water and transducers, since there was no interven- 
ing dome. However, such an arrangement does not 
permit the use of a stabilized transducer assembly. 
The problem of roll in the aircraft torpedo was even 
more serious than in the submarine type, so the use 
of such a stabilized array was considered essential. 
Lt. Col. Bree has indicated that the problem of roll 
stabilization of the aircraft-launched torpedo had 
been pretty well solved by the Luftwaffe at the end 
of the war although no Service torpedoes had been 
manufactured using this improved feature. 

The aircraft version of Geier differed from the sub- 
marine type in one other respect ; namely, the method 
of using the output signal of the amplifier to control 
the steering. Instead of disengaging the gyro and 
operating the steering engine by solenoid control, the 
signal caused the gyro to be angled by a small motor. 
When echoes ceased to be received, the gyro-angling 
motor stopped and the torpedo continued to run 
straight until another signal was received. Thus the 
aircraft-launched torpedo did not retain the original 
launching direction for its gyro course. 

23.3 GEIER 2 

The Geier 2 circuit differs from the Geier 1 prin- 
cipally in three features. The first principal difference 
is that the projectors are excited by a power ampli- 
fier driven from a conventional oscillator which is 
keyed from the time base. Although this arrange- 
ment requires more components than the simple con- 
denser discharge circuit of Geier 1 it has two distinct 
advantages over the latter system: First, the con- 
tacts on the time base which connect the projectors 
to the output amplifier are already closed when the 
oscillator begins, so that sparking is eliminated, and 
second, the transmitted pulse is much cleaner. The 
envelope is approximately square, as compared with 
the rapidly decaying exponential envelope produced 
by Geier 1. Because of better frequency control most 
of the transmitted energy is concentrated in a nar- 
row band around the nominal operating frequency. 
Not only is this more efficient than the damped- 
oscillation type of transmitter, it is also more secure, 
since the pinging can only be detected by listening 
in the neighborhood of the central frequency. 


144 


GEIER TORPEDO CONTROL SYSTEM 


The second principal difference is the addition of 
a noise discriminating circuit to the latter. The prin- 
ciple of this discriminator may be seen from the block 
diagram of Figure 6. After passing through four 
stages of amplification, the first three of which are 
controlled by a TVG circuit, the signal is passed 
through two filter channels. Channel A is a single 
band-pass filter whose mid-frequency is the trans- 
mitted frequency plus the average doppler shift ex- 
pected to be encountered. Channel B is a double 
band-pass filter, having high attenuation at the echo 
frequency, the two pass bands being located sym- 
metrically on either side. The output of each channel 


the echo signal must be of at least the same order of 
magnitude in order to provide a reasonable differen- 
tial between the two rectified voltages. However, in 
the relatively quiet periods between the noise bursts, 
a much smaller echo will be equally effective. Thus 
the circuit behaves as a rapid AVC circuit, with the 
added advantage that it can discriminate between 
noise and echo when the former has fluctuations com- 
parable with the echo duration. 

The third way in which Geier 2 differs from Geier 
1 is in the functioning of the control circuits (Veil- 
chen). Whereas the Geier 1 embodies symmetrical 
steering, Geier 2 employs preferred-side steering. 



Channel "B* 


Figure 6. Discriminator. 


is then rectified and these two rectified voltages are 
combined in opposite sense. The filter pass bands 
are so chosen that if white noise is fed into the system, 
the rectified voltages are equal and hence the result- 
ant is zero. However, if an echo signal is present, it 
appears only in the single pass channel so that the 
rectified voltages are no longer equal. If the differ- 
ential is greater than a predetermined threshold, the 
relay assembly is operated. The use of two pass bands 
in the noise channel permits the same result to be 
attained if the noise spectrum is not flat but has a 
constant slope in the part of the spectrum covered 
by the two channels. This noise discriminator is most 
effective against intermittent noise such as is pro- 
duced by explosions. While the noise signal is present 


The manner in which this is accomplished may be 
seen from Figure 7. This simplified diagram contains 
all of the essential features of the original control 
circuit shown in the Minerva Radio drawing of Feb- 
ruary 1, 1945. 58 

VI and V2 represent the output tubes of the port 
and starboard amplifiers respectively. Relay 1, relay 
2, and relay 4 are polarized differential relays. Cur- 
rent through winding 1-5 will operate the armature 
to the T position, and current through the 9-10 wind- 
ing will cause it to move to the Z position. The arma- 
ture remains on the side to which it was last moved 
after current ceases to flow. A differential of 0.3 ma 
is required between the 1-5 and 9-10 windings to 
cause the armature to move. Relay 3 is a spring- 


GEIER 2 


145 


controlled relay which is operated to the T position on 
0.6 ma through either winding and holds in on a cur- 
rent of 0.3 ma. When relay 3 is open, both the gyro- 
disengage solenoid and the steering solenoids are 
disabled. Relay 4 is the steering relay, causing either 
the port or starboard solenoid to be energized, when 
relay 3 closes. 

A thermal delay switch, which closes after the 
initial safety delay, is placed in series with the 200- v 
supply to the relay circuits. Finally, the biasing cir- 
cuit, controlled by the preferred-side switch, permits 
a steady current of 0.3 ma to be passed through either 
winding of relay 4. 


of relay 1, so that the armature is immediately re- 
turned to the Z position, charging Cl again. Simulta- 
neously C3 discharges through the 0.2-megohm resis- 
tor and winding 1-5 of relay 3. The operating current 
of 0.6 ma is reached during the very short interval in 
which C3 is being charged, but the minimum holding 
current of 0.3 ma is not reached until a considerable 
time afterward, on account of the long time constant 
of the discharge circuit. During this time interval, 
relay 3 is closed, so that the gyro is disengaged and 
the torpedo is under control of the steering solenoids. 
Since relay 4 is already in the port position, the addi- 
tional current which flows through winding 1-5 pro- 



Suppose now that the initial delay has elapsed, the 
thermal switch has closed, and the preferred-side 
switch is in the port position. Relay 3 is open and 
relay 4 is in the port position, since the preferred-side 
switch is set to this side. Relay 1 and relay 2 are 
both in the Z position, so that Cl and C2 are charged 
to 200 v. Suppose now that a signal arrives in the 
port channel. V3 fires, discharging Cl through wind- 
ing 1-5 of relay 1. The armature of relay 1 moves to 
the T position, and C3 is rapidly charged to the firing 
voltage of V5. When V5 fires, C3 begins to discharge 
through the 0.2-megohm resistor and winding 9-10 


duces no action, and the torpedo rudder is put hard 
aport. After 0.4 sec has elapsed, C3 has discharged 
sufficiently so that the current through winding 1-5 
of relay 3 drops below 0.3 ma; relay 3 then returns to 
the open position, the steering solenoids are disabled, 
and the torpedo returns to gyro control. 

Next, consider the sequence of events when an 
echo is received in the starboard channel. C2 is dis- 
charged, momentarily operating relay 2 to the T 
position, charging C4. C4 then discharges through 
the 9-10 winding of relay 2, operating relay 2 to the Z 
position and also through the 9-10 windings of relays 


146 


GEIER TORPEDO CONTROL SYSTEM 


3 and 4 operating relay 3 as before. The initial value 
of the current through 9-10 of relay 4 exceeds 0.6 ma, 
so that the differential between 9-10 and 1-5 of this 
relay is greater than 0.3 ma, and the armature moves 
to the starboard position. Thus the torpedo steers to 
starboard under solenoid control until relay 3 drops 
out, re-engaging the gyro. 

Thus far, the preferred-side control duplicates the 
action of the symmetrical control used in Geier 1. 
Now consider the behavior when echoes are received 
in both channels in the same listening interval. If the 
first echo is from port, relay 3 is operated and relay 

4 is moved to the port position, causing the torpedo 
to steer. A short time later, the starboard echo comes 
in, and current also flows through windings 9-10 of 
relay 3 and relay 4. Relay 4, however, remains in the 
port position because the current through the 9-10 
winding cannot exceed the total current through 
winding 1-5 by the required 0.3 ma until near the end 
of 0.4 sec hold-in time. In general this condition is 
not reached until after the next ping. On the other 
hand, if the first echo is received in the starboard 
channel, relay 4 is initially operated to the starboard 
position, but as soon as the port echo comes in, relay 
4 is reversed because the differential current in the 
two windings does exceed 0.3 ma. Thus the control 
“prefers” the port side, and causes the torpedo to 
steer to starboard only if no port echoes are received. 

23.4 COMPARISON OF SYMMETRICAL 
STEERING AND PREFERRED-SIDE 
STEERING 

A comparison was made on paper to determine the 
relative effectiveness of symmetrical steering and 
preferred-side steering. The width of the path within 
which the torpedo track had to lie in order to secure 
a hit was chosen as a measure of this effectiveness. 
This path width is a function of target dimensions, 
ratio of target speed to torpedo speed, direction pat- 
terns of the transducers, and track angle of the tor- 
pedo prior to the initiation of acoustic control. The 
meanings of these variables are illustrated in Figure 
8. The German analysis assumed that the maximum 
acoustic-control range for echoes received from the 
ship was 200 m, and for wake echoes 50 m. 

First, consider the qualitative behavior of the tor- 
pedo with the two types of steering. Suppose that the 
torpedo approaches the target on the port bow. If it 
is already on a collision course no echoes will be re- 


ceived until the last few yards of the run, on account 
of the narrowness of the transducer patterns and the 
wide angle between them. The acoustic control does 
not affect the performance. If the torpedo tends to 
miss ahead of the target, ship echoes will be received 
in the starboard channel, and the torpedo will steer 
on a curved track in such a way that it tends to lead 
the target by a continually decreasing amount as it 
comes closer. This procedure will result in a hit. 
Moreover, since echoes are received only in one chan- 
nel there is no difference between the symmetrical 
and preferred-side methods. 

The difference becomes apparent when the torpedo 
tends to miss astern. In this case the first echoes are 
received in the port channel, so that the torpedo 
turns in that direction. Eventually, the torpedo head- 
ing is at 90 degrees to the ship’s track, so that the port 
echoes from the ship and starboard echoes from the 
ship or wake arrive simultaneously. With symmetri- 
cal steering this condition is stable since, if the tor- 
pedo turns slightly to port, in the next ping interval 
the starboard echo comes in first and turns the tor- 
pedo back. When it has turned too far, the port echo 
arrives first and as a result the torpedo tends to run 
in to the ship or wake at right angles. Whether or not 
a hit is secured depends on the lateral distance from 
the ship’s track at which this condition is set up, and 
upon the speed ratio. For very close misses astern, 
the torpedo will be close to the ship’s track at the 


SHIP POSITION AT TIME 
TORPEDO IS ON LINE A A 



Figure 8. Diagram for steering analysis. AA = 
width of path within w r hich torpedo track must lie in 
order to secure a hit (no acoustic control). 


time it starts to run in on a perpendicular course, and 
a hit will be secured. For larger misses astern, the 
torpedo will merely cross the wake at right angles. 
Upon emergence, it will have lost acoustic contact, 
and will revert to gyro course, proceeding away from 
the target. 


STABILIZED TRANSDUCER ASSEMBLY 


147 


With preferred-side steering with the preferred- 
side switch set to port, the torpedo would not run in 
on a perpendicular course after arriving at a point 
where the port and starboard echo-ranges are equal. 
Instead, it would continue to maneuver so that the 
port transducer beam alternately cut on and off 
either the bow of the target or the most forward part 
of the target from which echoes were received. Thus 
the torpedo would pursue the ship, and in general, 
hits would be secured from initial torpedo tracks 
corresponding to larger misses astern than is the case 
with the symmetrical method. 

The improvement to be gained by using the pre- 
ferred-side method is much more striking when the 
track angle is greater than 90 degrees. In this case 
the first echoes always come from the starboard side, 
so that with the symmetrical system the torpedo will 
cross the wake at right angles for all approaches ex- 
cept those lying in a very narrow path which would 
yield misses ahead in the case of nonacoustic control. 
With the preferred-side steering this is not the case, 
as was explained in the preceding paragraph. 

By plotting out a number of torpedo tracks it is 
possible to ascertain the dependence of effective path 
width on track angle for the three cases of nonacous- 
tic control, acoustic control with symmetrical steer- 
ing, and acoustic control with preferred-side steering. 
This was done employing the following assumptions: 

Torpedo speed 24 knots 

Ship speed 12 knots 

Torpedo turning radius 75 meters 

Ship’s length 100 meters 

Ship’s beam 15 meters 

Maximum echo range (ship echoes) 200 meters 

Maximum echo range (wake echoes) 50 meters 

The results are shown in Figure 9A. The average 
path widths for the three cases in the two 90-degree 
sectors forward and aft of the beam are shown in 
Figure 9B. From this comparison the following con- 
clusion may be drawn: Both the symmetrical and 
preferred-side methods provide a considerable in- 
crease in performance over the nonacoustic torpedo 
for shots in the bow sector. There is little difference 
in performance between the two methods. In the 
quarter sector, however, the symmetrical method is 
only slightly better than no acoustic control, but the 
preferred-side method gives a marked improvement. 
The preferred-side method has one fault which does 
not appear in the foregoing analysis: If the target 
detects the torpedo in sufficient time to take com- 


plete avoiding action; i.e., to bring the ship head-on 
or stern-on to the torpedo, then the latter may be on 
the wrong side of the target when it comes under 
acoustic control. In this event the torpedo will at- 
tempt to steer on the tail of the wake, instead of on 
the ship. For this reason the estimated effectiveness 
of the preferred-side method should be reduced from 
the values indicated in the foregoing comparisons. 

According to the statement of the Atlas Werke 
engineer who spent most time at the trials in Gdynia, 
the bulk of these experimental shots was made with 
symmetrical steering against targets moving at 
speeds of 10 to 15 knots. The observed performance 
was in good agreement with the theoretical analysis, 
at least in so far as the major items were concerned. 
Shots made on the quarter almost always resulted 
in misses astern because of the torpedo’s attempt to 
cross the wake at right angles. Bow shots, however, 
were successful. A few of the Geier 1 units were 
equipped with a preferred-side control. Trials with 



0 30 60 90 120 150 ISO 

A TRACK ANGLE IN DEGREES 



B PATH WIDTH IN METERS 

Figure 9A, B. Steering analysis curves. 


these torpedoes tended to confirm the generalization 
that this type of steering was equally good for all 
track angles, provided the “preferred” side was prop- 
erly chosen. Because of this superiority, the preferred- 
side control was incorporated in Geier 2. 


23.5 STABILIZED TRANSDUCER 
ASSEMBLY 

The self noise of the modified G7e torpedo in which 
Geier 1 was installed is said to be roughly equal to 
the reverberation level of the Geier signal corre- 
sponding to an echo range of 200 m. Hence it was 


148 


GEIER TORPEDO CONTROL SYSTEM 


important to take all possible measures to minimize 
reverberation. It was found in the sea trials that ex- 
cessive roll of the torpedo caused a marked increase 
in reverberation which seriously impaired the per- 
formance. This was to be expected on account of the 
wide angle between the two projector beams; a slight 
roll would throw most of the energy from one pro- 
jector up into the surface. A similar difficulty, 
though not so great, resulted from pitching of the 
torpedo. In order to meet this problem, a stabilized 
transducer assembly, Pendel-Rose, was devised. 

In this mechanism the four transducers are 
mounted on a framework suspended on ath wart- 
ships bearings. A large piece of lead is attached to 
the bottom of the assembly making it pendulous. 
This feature tends to keep the transducers stable 
with respect to pitch. The pendulum bearings are 
secured to a plate behind the transducers. This plate 
is capable of axial rotation, and is driven by a small 
motor. On the back of the plate are mounted two 


small mercury switches slightly inclined in opposite 
directions to the horizontal. When there is no roll, 
i.e., the transducer faces are vertical, both of these 
switches are open and the motor is stopped. When 
the roll exceeds 5 degrees, the mercury in one of the 
switches moves, closing a circuit which causes the 
motor to drive the plate around, reducing the roll 
to less than 5 degrees. The entire mechanism is very 
simple. It was stated that no electric interference in 
the signal channels was produced by the motor. The 
actual performance of the stabilized transducer as- 
sembly in reducing reverberation due to roll and 
pitch had not been extensively tested, but it was in- 
tended to use this feature in future Geier 2 units. 
Furthermore it was stated that such an assembly 
capable of withstanding short-period accelerations 
of 1,000 times gravity had been designed and tested 
for the aircraft torpedo. 8 

a See references 58-64 for additional material on topics in 
this chapter. 


Chapter 24 

BRITISH TRUMPER SYSTEM 


24.1 INTRODUCTION 

T he British Trumper torpedo is a prosubmarine 
anti surface-ship device, which was developed 
for use in the Mark 9 torpedo. The Mark 9 is a 21-in. 
torpedo that travels at a speed of 40 knots and has 
a turning radius of about 125 yd. 

The block diagram of the Trumper control system 
is shown in Figure 1. The transducers used are quartz 


course, the horizontal beamwidth of the transducers 
is used as the means of making initial acoustic con- 
tact with the target. The beamwidth of the trans- 
ducers in the vertical plane is made very narrow in 
order to achieve a high directivity index. Diagrams 
of the quartz transducers for both the projectors and 
the receiving hydrophones are shown in Figure 2. 
The following are the characteristics of these trans- 
ducers. 



Figure 1 . Block diagram of British Trumper. 


sandwich type with separate transducers used for 
transmitting and receiving. The time base is a system 
of cams driven by the main motor shaft. These cams 
determine the 1-sec interval between transmitted 
pulses, the 3- to 5-msec length of transmitted pulses, 
the blanking, and application of TVG to the receiver 
channels. 

24.2 TRANSDUCERS 

The transducers are mounted in a flattened section 
on the nose of the torpedo. Since the torpedo is fired 
under gyro control to follow a straight gyro search 


1. Projectors. The vertical pattern is 6 db down 
at 10 degrees off the axis and 10 db down at 16 de- 
grees off the axis. The minor lobes are 13.5 to 14.5 
db down and the first minor lobes are 23 degrees off 
the axis. The horizontal patterns for one-half the 
transducers are 5 db down at 45 degrees off the axis 
and 14 db down at 70 degrees off the axis; the minor 
lobes are negligible. When the projectors are con- 
nected with halves aiding, the 6-db down points are 
25 degrees off the axis, the 10-db down points are 31 
degrees off the axis, and the first minor lobes are 63 
degrees off the axis and are 20 db down. When they 
are connected with halves bucking, the pattern is 


149 


150 


BRITISH TRUMPER SYSTEM 


bi-lobed with a zero on the torpedo axis. The angle 
between the maxima of the lobes is 60 degrees, the 
pattern is 12 db down at ± 70 degrees off the torpedo 
axis and 10 db down at + 5 degrees off the torpedo 
axis. 

h-i 




4 ' 


FrtOTEC TO/? cf/ 1 //A/G 

//YD/?0T>//0//E 

Figure 2. Transducers. 

2. Hydrophones. The horizontal pattern of the 
receiver hydrophones is almost identical with that of 
the projectors, whereas the vertical pattern is about 
twice as wide. 


24.3 TRANSMITTER 

The diagram of the transmitter circuit is indicated 
in Figure 3. The oscillator VI which operates from a 
plate supply of 300 v is not keyed by the time base 
but operates continuously. The driver stage V2 is 
driven by means of the oscillator and its plate circuit 
is keyed by means of a switch in the time base. The 
driver stage is coupled to the power amplifier by 
means of the transformer Tl, the two secondary 
windings of which are so arranged that the grids of 
the power amplifier stages V3 and V4 are driven 180 
degrees out of phase. The power amplifier is con- 


nected to the two projectors by means of the trans- 
former T2. The power-amplifier stage is keyed by 
means of a switch SW2 operated by the time base. 
This switch breaks the circuit between the cathodes 
of V3 and V4 and ground. The electric output of the 
power amplifier is about 300 watts and the efficiency 
of the projectors is such that about 200 watts of 
acoustic power is radiated into the water. The power 
supply for the power amplifier is mounted in a cylin- 
der about 10 to 12 in. long and 3 in. in diameter and 
consists of a high-voltage 1,500-c generator which is 
driven by means of an air turbine. The output of the 
generator is rectified and supplied directly to the 
power-amplifier plates. 

24.4 STEERING RECEIVER 

The steering receiver consists of a four-stage 
resistance-coupled amplifier which is shown schemat- 
ically in Figure 4. The first stage is considered as a 
preamplifier and no sensitivity control is applied to 
it. The second and third stages are blanked during 
transmission and are controlled by TVG during the 
listening interval. The circuits containing R9, P2, 
C7, and C8 control the blanking and TVG voltages. 
During transmission a high negative voltage is ap- 
plied at the junction of R8 and R9. The circuit C7 
and R9 serves as a fast-discharge circuit which allows 
the potential of the grids of V2 and V3 to drop quite 
rapidly immediately following transmission. The cir- 
cuit consisting of P2 and C8 is a slow-discharge cir- 
cuit which actually controls the TVG during the 
major portion of the listening interval. By this ar- 
rangement a voltage high enough to achieve blanking 
of the receiver during transmission is applied at the 
junction of R8 and R9, but very soon after trans- 
mission this voltage drops to the proper level for 
control of the receiver sensitivity during the listening 
interval and the rate of change of the sensitivity is 
then controlled by the circuit P2 and C8. The poten- 
tial of the grid of V3 is maintained by way of R15 
and the potential of the junction of R9 and P2. The 
entire receiving system contains two identical re- 
ceivers like that shown in Figure 4 with the TVG 
circuits of these two receivers connected together as 
indicated in Figure 4. The inputs of these two re- 
ceivers are obtained from two separate identical re- 
ceiving hydrophones. 

The fourth stage of the receiver is biased beyond 
cutoff by means of a negative voltage applied to its 



STEERING RECEIVER 


151 


/ o 

mh 



S6. OOO - n - 


Figure 3. Transmitter. 



grid by way of P3, R20, and R21. When a signal is 
received on the hydrophone, this negative bias on V4 
serves as an amplitude gate for the system, requiring 
that a signal level out of V3 be higher than a prede- 


termined value in order that any signal be generated 
in the plate circuit of V4. This stage also serves as a 
limiter stage since a level of about 3 db above the 
threshold signal level is required to produce limiting. 


152 


BRITISH TRUMPER SYSTEM 


The phase-sensitive detector circuit which is used re- 
quires that the applied signals be out of phase by 90 
degrees in order to produce a zero output. This 
90-degree phase difference in the two receivers is 
achieved by proper tailoring of the coupling con- 
densers C4, Cll, and C16 in the two receivers. 


24.5 PHASE-SENSITIVE DETECTOR 
AND RELAY CONTROL 

The schematic of the phase-sensitive detector cir- 
cuit is shown in Figure 5. The outputs of the two re- 
ceivers appear at the transformers T1 and T2. The 



phase shifts in the receivers are adjusted by tailoring 
the coupling condensers so that for signals in phase 
applied at the receiver inputs the signals at the out- 
put of the receivers will be 90 degrees out of phase 
with each other. Figure 6 is a curve indicating the 
d-c voltage output of the phase-sensitive detector 
plotted as a function of target angle in degrees. The 
values of the phase-sensitive detector output volt- 
ages are expressed as fractions of the signal voltage 
generated in the plate circuits of the limiter stages. 
The maximum output voltage occurs at a target 
angle of 30 degrees and is equal to 0.7 of the value 
of the limiter signal voltage. The output of the phase- 
sensitive detector appears at the terminals of the two 
condensers Cl and C2 connected in series. The ter- 
minal of Cl is connected to the grid of a d-c ampli- 
fier which operates one steering relay and the other 
terminal of C2 is connected to another d-c amplifier 
which operates another steering relay. These steering 
relays are used to rotate the gyro by means of a gyro- 
angling motor. When a steering signal is received, 
one or the other of the relays is closed and this causes 


the gyro-angling motor to change the setting of the 
gyro by 7 degrees. This amount of correction of the 
gyro per ping is chosen since it is the rate at which 
the torpedo is able to turn under the application of 
full rudder. The relay amplifiers are so biased that 
the voltage required to actuate the relays corresponds 
to the phase-sensitive detector output at a target 
angle of 3 degrees. 


24.6 COLLISION-COURSE STEERING 

The British have done some work on a modifica- 
tion of the steering control system in order to permit 
the torpedo to be steered on a collision course. The 
modification of the steering circuit required to 



-80 -60 -40 - 20 0 20 40 60 60 

TARGET ANGLE IN OEGREES 


Figure 6. Phase-sensitive detector characteristics. 


achieve this is indicated in Figure 7. S is a stepping 
relay which is used to change the relative phase- 
shifts of signal in the two steering amplifiers. The 
relays, indicated as relays 2, are normally closed, but 
they can be driven open by a signal of somewhat 
higher level than that required to close the relays 1. 
When a signal is received which requires steering in 
one direction but which produces a phase-sensitive 
detector output sufficient to actuate relay 1 but not 
to actuate relay 2, the gyro-angling motor will in- 
troduce the correction of 7 degrees in the course in 
the 1-second interval between echoes. At the same 
time the stepping relay S will be turned to introduce 
phase shift in the opposite sense between the two 
amplifiers amounting to 4 degrees which is the dif- 
ference between the angle of correction introduced by 
the gyro-angling motor and the minimum sensitivity 
of the relay amplifiers. By this means the torpedo will 


COLLISION-COURSE STEERING 


153 


asymptotically approach a course in such a way that 
the bearing of the target relative to the torpedo will 
remain constant. This is by definition the collision 
course. If the target angle at the time of initial con- 


S from operating. This condition will be continued 
until the torpedo is near enough to a pursuit course 
so that relay 2 will be unactuated when the torpedo 
will begin to correct to the collision course. It is not 



Figure 7. Block diagram of collision-course system steering. 


tact is very large, the time which would be required 
to correct to the collision course would be excessive, 
so the relays 2 are provided to make the torpedo 
initially correct to a pursuit course under conditions 
of very large target angle where the signal applied 
to the relay amplifier is sufficiently large to actuate 
relay 2. Opening relay 2 prevents the stepping relay 


known whether any torpedoes were ever operated 
with the collision-course system. The initial plan was 
to proceed with the simple steering system and to in- 
corporate the collision-course system when it was 
sufficiently perfected. a 

a See references 65 and 66 for additional material on topics in 
this chapter. 



Chapter 25 

BRITISH BOWLER SYSTEM 


25.1 INTRODUCTION 

T he British Bowler torpedo utilizes an echo- 
ranging control system with two independent 
projectors and hydrophones. The system components 
were designed for aircraft launching. The projectors 
and hydrophones are so mounted that one projector 
transmits a beam out perpendicular to the torpedo 
axis on one side and a hydrophone placed beside it 
receives echoes from any target approximately to the 
torpedo axis. The other projector and hydrophone 


of 0.16 sec. The projectors and hydrophones are both 
quartz transducers of about 3-in. diameter. The pro- 
jectors are internally mounted and are acoustically 
coupled to the hull by means of an oil cell. The hy- 
drophones are externally mounted and are isolated 
from the hull of the torpedo. An interesting fact ob- 
served in connection with the mounting of the hydro- 
phones is the fact that the width of the annular space 
between the diaphragms of the hydrophone and the 
hull of the torpedo is critical. This annular space fills 
with water in the course of the torpedo’s run and the 


PORT PORT 

PROJECTOR HYDROPRONP 



STSD STS 3 

PROJPCT/OR RYOROPRO/YT 


Figure 1 . Block diagram of British Bowler system. 


are located similarly on the opposite side of the tor- 
pedo. The torpedo is normally launched from a bow 
or stern aspect and if the torpedo misses the target, 
an echo will be received on one or the other of the re- 
ceiving hydrophones, causing the torpedo to go into 
a hard turn toward the target. This arrangement re- 
quires that the range of the target be less than the 
diameter of the turning circle of the torpedo in order 
for it to make a hit. 

A block diagram of the system is shown in Figure 
1. The frequency of the transmitted signal is 26.7 kc; 
the length of the pulse is 2 msec with a ping interval 


optimum value for the width of the space is 0.040 in. 
A greater width than this apparently introduces tur- 
bulence causing increased self noise, while a narrower 
width permits the water to form a shunt path across 
the isolation. 

25.2 ELECTRONIC GEAR 

The transmitter consists of a blocking oscillator 
employing a 6N7 tube. The length of the pulses and 
the interval between pulses are controlled by the 
characteristics of this blocking oscillator. The oscilla- 


154 




PERFORMANCE 


155 


tor drives the power amplifier which in turn drives 
the projectors through a coupling transformer. The 
power output of each projector is 15 watts. Since the 
blocking oscillator is its own time base, it is neces- 
sary that this oscillator be used to control the blank- 
ing and the TVG of the receiver. These functions are 
achieved by means of the double-rectifier system in- 



Figure 2. Details of blanking and TVG circuits. 


dicated in Figure 2. The virtue of this system is that 
it achieves blanking of the second stage of the re- 
ceiver amplifier by a combination of increased bias 
on the grid and decreased value of resistance to 
ground from the grid without introducing transients 
into the system. The storage of charge on the con- 
denser C by the rectifier VR1 is used for the TVG 
control. The receivers consist of two stages of resist- 
ance-coupled amplifiers followed by a single gas- 
discharge tube which is used to control the steering 
action. Considerable difficulty was encountered in 
obtaining control of the rudders by means of sole- 
noids operated by relays from the gas tubes because 
space did not permit installation of adequate sole- 
noids. The final solution of this problem was quite 


unusual and was possible only because the torpedo 
is intended to have steering action applied just once 
during the course of an attack. The device consisted 
of two small cylinders each containing an explosive 
charge back of a piston. When an echo is received, 
indicating that the torpedo should turn in one direc- 
tion, firing of the gas tube in the receiver causes the 
charge in the cylinder to be fired which in turn throws 
the rudder hard over in the proper direction. It is 
impossible for any further steering to take place 
until the torpedo is recovered and the cylinders are 
reset. 

The size of the complete electronic chassis is ap- 
proximately 5 x 6 x 12 in. The power supply con- 
sists of a 12-v Edison storage battery which is 
12 x 4 x 3 in. The high-voltage plate supplies for 
the tubes are obtained by means of vibrators with 
transformers and rectifiers. 

25.3 PERFORMANCE 

The dynamic sound pressure during transmission 
is between 2,000 and 6,000 dynes per sq cm at a 
range of 10 ft on the axis of a projector. Echoes of 
50 to 100 dynes per sq cm have been obtained at a 
range of 100 yd off the beam of a stationary tanker. 
Echoes off the bow of such a ship at 100 yd range are 
approximately 20 to 30 dynes per sq cm. The peak 
noise level of the torpedo operating at a speed of 40 
knots is from 5 to 8 dynes per sq cm. The signal-to- 
noise ratio under the most unfavorable conditions at 
100 yards range is about 12 db. The average value is 
more nearly 20 db. a 

a See references 66-68 for additional material on topics in 
this chapter. 


Chapter 26 

EVALUATION 


A ll of the echo-ranging control systems which 
have been described in the preceding chapters 
are systems which were developed under the stress 
of war with the chief objective to get a working de- 
vice in the shortest possible time. In all cases com- 
promises had to be made in order to avoid discarding 
something already developed and taking the neces- 
sary time to go back and re-engineer parts of systems 
which were found unsatisfactory. This resulted in 
most of the devices being made more complicated 
than necessary and containing components which 
are forced to operate under marginal operating con- 
ditions. The problem of maintenance and adjust- 
ment of the systems is in all cases more complicated 
than should be necessary. 

In the case of the antisubmarine devices developed 
in this country, a device was desired which could be 
incorporated in the existing torpedo body already 
being used as a noise-steering torpedo. Since the body 
used in this torpedo is not capable of withstanding 
pressures corresponding to depths greater than about 
400 ft, there was little immediate advantage to be 
gained in designing the electronic gear with a view 
to operating over a greater range of depth. The sys- 
tem developed by General Electric and engineered 
for production by the Leeds and Northrup Company 
fulfilled the requirements for this device in a quite 
satisfactory manner. It is one of the simplest echo- 
ranging systems which has been developed. The chief 
weakness of the system is the fact that the rate of 
dive and climb has to be quite severely limited. The 
maximum climb angle permitted is about 1.5 degrees 
which provides relatively little maneuverability in 
the vertical plane if a submarine under attack takes 
evasive action in the vertical plane. This is not a 
serious limitation when the device is applied to a 
body which is restricted to the upper 400 ft of water 
and is aircraft-launched against the swirl left by a 
diving submarine. If, however, the device is applied 
in a torpedo capable of operating over the range of 
depth to which the most modern submarines can 
operate, namely, about 1,000 ft, it is quite possible 
that evasive tactics in the vertical plane on the part 
of the submarine would be successful in evading the 
torpedo. 


The General Electric NO 181 system utilizes trans- 
mitter pulse lengths of about 30 msec with an am- 
plitude-gate characteristic which prevents the device 
from steering on short pulses. This makes the system 
quite invulnerable to grenade-type countermeas- 
ures. The cut-on cutoff steering employed in azimuth 
makes the device quite invulnerable to noise counter- 
measures towed astern or thrown out from a sub- 
marine under attack. The cut-on cutoff steering sys- 
tem causes the torpedo to steer around the noise 
source, and if echoes are picked up on the far side 
again, the torpedo will steer on the echoes from the 
target. The device is, however, vulnerable to a decoy 
in the form of a very strong noise source at the target. 
This form of decoy will cause the torpedo to steer 
around the target rather than attack it. 

The behavior of this system on wakes is quite in- 
teresting. The cut-on cutoff steering system in the 
azimuth plane causes the torpedo to steer parallel to 
the wake along one side. If the device steers on the 
wake, it will follow the wake to the target if started 
in the proper direction. In antisubmarine application, 
this is an advantage, since the torpedo when aircraft- 
launched is normally launched as near as possible to 
the swirl left by the diving submarine. This wake- 
following feature might well be a disadvantage in an 
anti surface-ship device, since in this case the torpedo 
is normally launched to run toward the target. The 
acoustic contact with the wake is likely to be such 
that the torpedo will follow the wake away from the 
target. 

The Harvard N0181 system is a considerably 
more complicated device than the General Electric 
device. It utilizes target doppler for enablement, and 
in addition, uses an amplitude gate with a 30-msec 
transmitted pulse. This system is somewhat less vul- 
nerable to countermeasures than the General Electric 
system and, in addition, high rates of dive or climb 
can be used since the device will not steer on surface 
or bottom echoes. The doppler system also prevents 
the device from steering on wake echoes. In a body 
with the limited capabilities of the one into which 
this device was built, there is some question as to 
whether the advantages gained by the additional 
complication of the doppler-enabling system were 


156 


EVALUATION 


157 


worth while. However, in the development of an 
echo-ranging antisubmarine torpedo for operation at 
depths as great as 1,000 ft where vertical evasive 
tactics on the part of the submarine can become im- 
portant, the additional rate of dive and climb per- 
mitted by the doppler-enabling system probably will 
become an important feature. This system is also in- 
vulnerable to grenade-type countermeasures because 
of the action of the amplitude gate. There is some 
tendency for the device to steer toward a noise 
source, so if the target is made a source of noise, the 
noise would simply aid the device in steering on the 
target. The behavior of the device in the presence of 
a continuous-noise source towed by the target or 
thrown out from the target is similar to that of the 
General Electric system. The torpedo might steer 
toward the noise source but after passing through it, 
it would be free to search on the target again; the 
sharp beam pattern of the transducer minimizes the 
effect of the countermeasure after the torpedo has 
passed. It should be noticed, however, that the use 
of a doppler-enabling system makes possible a simple 
evasion tactic, since it makes steering on a stationary 
submarine impossible. 

The British Dealer torpedo was also designed as 
an echo-ranging antisubmarine torpedo, but rela- 
tively little is known of the nature of the electronic 
gear used. The methods used in the actual steering of 
the torpedo avoid the use of rudders which have to 
be operated through watertight seals on the body. 
The result was that two propulsion motors and two 
propellers are used and a means of moving the bat- 
tery backward and forward is provided in order to 
steer the torpedo in the vertical plane. 

All these antisubmarine torpedoes were designed 
so that they could be aircraft-launched. This placed 
relatively severe requirements on the design of all 
components entering into them. 

The echo-ranging anti surface-ship torpedo which 
is simplest in principle and has the most limited ob- 
jective is the British Bowler. This device is intended 
to have an acoustic operating range less than the 
turning diameter of the torpedo and it is intended 
that when the device receives an echo from one side, 
the rudders will be turned hard over and the torpedo 
will simply turn into the target. The transducers for 
the two separate transmitting and receiving systems 
are mounted on the two sides of the torpedo with 
their acoustic axes almost perpendicular to the axis 
of the body. The purpose of a device such as this is 
to increase the effectiveness of torpedoes launched off 


the bow or stern of the target ship, but for large 
misses the acoustic control system does not add any- 
thing to the effectiveness of the torpedo. 

One of the simplest of the anti surface-ship torpedo 
echo-ranging systems is the Geier 1 developed by the 
German Luftwaffe. The objectives of this device are 
somewhat limited since its operating range is only 
about 200 meters. This device also uses two inde- 
pendent sets of transducers, and acoustic control is 
inaugurated when an echo is received on one of the 
receiving hydrophones. However, the device differs 
from the Bowler device in that acoustic control is 
maintained until the torpedo strikes the target. This 
device is quite vulnerable to countermeasures, since 
it operates on any sudden change in signal level. It 
will not steer, however, on a continuous-noise coun- 
termeasure, because of the action of the special 
A VC circuit. It is also susceptible to steering on tar- 
get wakes. There is an important difference between 
the behavior of this system and the General Electric 
system with respect to the target wake. The cut-on 
cutoff steering of the General Electric device causes 
it to steer parallel to the wake while the behavior of 
the German Geier system is such that when it steers 
on echoes from the wake, it tends to steer perpen- 
dicular to it. In any case, when the torpedo gets in 
such a position that it is closer to the wake than it 
is to the target, it will steer perpendicular to the 
wake. The Germans were doing considerable work 
toward improvement of the performance of this de- 
vice in order to eliminate its vulnerability to the 
wake. The Geier 1 system was not intended to be 
used as a Service system but was simply intended to 
be used as a sort of guinea pig in the development of 
an improved device. 

The Bell Telephone Laboratories [BTL] 157C 
system and the British Trumper systems are quite 
similar in their plan of operation. Both of these de- 
vices use a split-hydrophone system and the elec- 
tronic gear in the receiver compares the phase of the 
signals on the two halves of the receiving hydro- 
phones. In both cases the hydrophones and projec- 
tors are crystal. The British system uses quartz crys- 
tals and the BTL system uses ammonium dihydrogen 
phosphate crystals. The BTL device uses the same 
transducer for both projector and receiver. In the 
BTL device the target angle is actually measured by 
comparison of phase of signal on the two halves of 
the transducer and the measured value of the target 
angle plus a small correction angle is injected to the 
gyro cam plate by means of a mechanical device 


158 


EVALUATION 


called a translator. In the British system an incre- 
ment of angle is injected to the gyro cam plate when- 
ever the acoustic signal indicates that the target is 
off the axis. On the basis of an analysis made by the 
BTL group, the system used by the British should 
be just as effective and it is capable of being made 
considerably simpler. One of the important criticisms 
of the BTL device is that in many ways it is exces- 
sively complicated. For example, the time base of the 
system consists of three double triodes and two re- 
lays, where the same operations can be performed by 
a simple set of cam-operated switches. The system 
whereby the target angle is measured also requires 
that a condenser and considerable circuit network 
have to be isolated from ground by a resistance of the 
order of 100 megohms. This is a requirement which 
probably cannot be maintained under all expected 
conditions of operation. 

The German Geier system, the British Trumper, 
and the BTL system all suffer from the same diffi- 
culty with target wakes. The solution of the difficulty 
by the Germans and the BTL group is quite similar 
and involves use of a preferred-side steering. This is 
accomplished by setting a switch, operable from the 
outside of the torpedo, which causes the system to 
steer in the preferred direction when echoes are re- 
ceived in a listening interval which give steering in- 
formation in both directions. With this arrangement 
the submarine skipper determines, before firing the 
torpedo, on which side of the target the torpedo will 
approach, and the preferred-side steering system is 
so set up that for that side of approach to the target 
the torpedo will prefer echoes from the target. In 
these systems it is assumed that the target will pur- 
sue the course estimated at the time of firing. If the 
torpedo is fired from a long running range and the 
ship is executing evasive maneuvers, there is a possi- 
bility that the ship will be in such a position that 
the torpedo will not be on the preferred side. In this 
case, the torpedo will prefer echoes from the wake 
and will be actually less likely to contact the target 
than it would have been without the preferred-side 
steering. The British also encountered another dif- 
ficulty with their system in the presence of wakes. 
The broad transducer pattern in the azimuth plane 
causes the phase-sensitive detector to receive a long 
signal of continuously-varying phase which inter- 
feres with its action to such an extent that they were 
considering a change from a phase-comparison sys- 
tem to an amplitude-comparison system. 

The Ordnance Research Laboratory [ORL] proj- 


ect 4 device is an outgrowth of the Harvard NO 181 
system with the emphasis on antisurface-ship appli- 
cation rather than antisubmarine application. Since 
the problem of target wakes is a much more impor- 
tant problem in anti surface-ship applications, the 
doppler-enabling system gives this device consider- 
able advantage, since it quite effectively eliminates 
the wake-steering problem. The preliminary results 
which have been obtained using the special trans- 
ducer, which was developed with this system, indi- 
cate that the effective self noise of the torpedo in a 
system using this transducer is considerably less than 
the effective noise level with other types of trans- 
ducer. The use of this transducer which has a very 
sharp beam pattern requires the snaky gyro-course 
for search in order to make acoustic contact with a 
target at an appreciable angle with the original gyro 
course of the torpedo. 

Relatively little attention has been paid the coun- 
termeasure problem in the echo-ranging systems 
which were developed during the war. The fact that 
countermeasures for echo-ranging systems are, in 
general, different from those effective against a noise- 
steering device was depended on to make former de- 
vices effective. In future development, the fact that 
countermeasures, designed to operate against echo- 
ranging torpedoes, will be used, will have to be con- 
sidered in the design of the systems themselves. The 
experience with the Harvard NO 181 and the Gen- 
eral Electric N0181 devices indicate that the use of 
relatively long transmitted pulses makes a system 
less vulnerable to countermeasures. In addition, 
methods of processing the received echoes will need 
to be devised in such a way that the criterion for 
steering can be varied from one unit to another so 
that when one type of countermeasure becomes ef- 
fective, the system can be changed to make this 
countermeasure ineffective. 

Since the self-noise level of the torpedo as meas- 
ured on the receiving hydrophone determines the 
lowest level of received echo which can be effective 
in steering, the design of the transducer to minimize 
self noise is important. So far, the experience with 
the transducer which is used in the Harvard NO 181, 
the General Electric N0181, and the ORL project 
4 systems indicates that this transducer, which has 
a very sharp beam pattern and is mounted in the 
center of the nose, measures a lower level of self noise 
from the torpedo than is measured by broader beam- 
pattern transducers and transducers which are 
mounted at points in the nose not at the center. It is 


EVALUATION 


159 


not yet known whether this difference is due to the 
more effective front-to-back discrimination of the 
transducer used or to the fact that the small trans- 
ducer mounted at the center of the nose is less af- 
fected by water-flow noise. Future investigations on 
the factors determining torpedo self noise should 
clarify this matter and make possible the more in- 
telligent design of transducer systems. 

In the antisurface-ship applications, some acous- 
tic torpedoes provide for only azimuth control and 
the running depth of the torpedo is so set that it can 
make mechanical contact with the target or, if an 
influence exploder is used, the torpedo will pass close 
enough under the target to actuate the influence ex- 
ploder. Experience so far indicates that self noise due 
to cavitation is decreased as the running depth of the 
torpedo is increased. Experience with the Mark 21 
and Mark 31 noise-steering torpedoes which use the 
Mark 13 and Mark 18 bodies indicates that self-noise 
level decreases with increased depth down to a depth 
of about 50 ft. The decrease in self noise is of suf- 
ficient magnitude so that it is worth while operating 
the torpedo at the 50-ft running depth for the initial 
portion of an attack and then to use a vertical- 
steering system to bring the torpedo up to a shallow 
enough depth to make the attack effective at the 
end. The experience so far with the Mark 28 noise- 
steering torpedo indicates that operation as deep as 
80 ft is desirable in the initial portion of the attack. 
Unless it is possible to design propellers which are 
entirely free from cavitation, it will probably con- 
tinue to be profitable to operate torpedoes at these 
greater depths during the initial portion of the at- 
tack. 

All torpedo echo-ranging systems so far developed 
have relatively complicated electronic gear which is 
so designed that the adjustments of the components 
of the system are critical, requiring quite highly 
skilled maintenance personnel at any station where 
the devices are made ready for operation. This con- 
dition need not always be true. With proper engi- 
neering the devices should be so worked out that, 
after adjustment is made in the factory, no further 
adjustment of any electronic components would need 
to be made in the field. It would be desirable to de- 
sign the electronic panels so that they contain two or 
three replaceable units with all components well pro- 
tected against mechanical injury and then supply 
test equipment for the field which will determine 
whether these unit components are operating prop- 
erly or not. If a component is found not to be oper- 


ating properly, it should be removed, replaced by 
another, and the defective component either dis- 
carded or returned to the factory for adjustment. 
This would greatly simplify the Navy personnel 
problem. 

One of the most important needs in the develop- 
ment of any type of acoustic torpedo is the develop- 
ment of a torpedo body with a control system to 
which the information from the electronic panel can 
be easily applied. It would seem that an all-electric 
control system is preferable to the air control sys- 
tems in current use on most torpedoes. In the case 
of the BTL device, a proportional control in azimuth 
is achieved by means of an extremely complicated 
mechanism to transfer the acoustic-steering informa- 
tion to the torpedo gyro cover plate. Use of an all- 
electric control system would make possible a very 
much simplified means of transferring this informa- 
tion to the gyro. In the case of the U.S. Navy Mark 
20 torpedo, which was never used in the Service, the 
control system for both depth and azimuth utilizes 
selsyns to transfer the information from the gyro to 
the torpedo azimuth-control system and from the 
pendulum and bellows to the depth-control system. 
With this type of control it is possible, by the intro- 
duction of another selsyn in the depth system and 
also another selsyn in the azimuth system, to inject 
the correction information from the steering ampli- 
fier to the normal torpedo control. One of the diffi- 
culties in the Mark 20 is the fact that steering motors 
are used, and trouble has been experienced with the 
stability of this torpedo because of the sluggish ac- 
tion of the steering motors. 

In order to achieve maximum range for an echo- 
ranging torpedo, it is necessary to have the ping in- 
terval great enough so that the transmitted signal 
can get to the target at the maximum range and back 
before the next transmitted signal. As the maximum 
range of the echo-ranging torpedoes increases, the 
information will become more intermittent and the 
difficulties in the use of an on-off steering system will 
become greater. This means that either a propor- 
tional azimuth-steering system as is used by BTL or 
an incremental azimuth system as is used in the 
British torpedoes will need to be used. 

One of the chief advantages of an echo-ranging 
system is the fact that the range achieved is quite 
independent of the nature of the noise emitted by the 
target. In order to utilize this advantage to the maxi- 
mum, large transmitter power outputs are essential. 
At the present time, the BTL NO 181 system and the 


160 


EVALUATION 


ORL project 4 system use transmitters of 1.5-kw 
capacity which is probably not the largest power out- 
put feasible in an echo-ranging device. The utiliza- 
tion of power in a duty cycle needs some further 
study. In the above mentioned systems, the actual 
power output is determined by the plate voltage 
limitations of the tubes used in the power amplifiers. 
The power-supply problem actually resolves itself 
into the problem of transforming power from the 
propulsion motor of the torpedo into useful power 
on a duty-cycle basis for the transmitter without the 


use of equipment which has an excessive weight or 
volume. In neither of the above systems is the power 
output of the transducer great enough to cause cavi- 
tation at its face during the transmitted pulse. 

As the acoustic range of torpedoes is increased, the 
running range of the torpedo should be increased in 
a corresponding manner in order to utilize fully the 
advantages to be gained by acoustic control. This is 
another reason why attention should be given to the 
design of torpedoes for use with acoustic control 
systems. 




GLOSSARY 


Acoustic Frequencies. Sonic frequencies, range of audible 
frequencies, sometimes taken as from 0.02 to 15 kc. 

Cavitation. The formation of vapor or gas cavities in water, 
caused by sharp reduction in local pressure. 

Ceiling Switch. Pressure-actuated switch which keeps con- 
trol system inoperative until torpedo exceeds some selected 
depth. 

Crystal Transducer. A transducer which utilizes piezo- 
electric crystals, usually Rochelle salt, ADP, quartz, or 
tourmaline. 

Directivity Index. A measure of the directional properties 
of a transducer. It is the ratio, in decibels, of the average 
intensity, or response, over the whole sphere surrounding 
the projector, or hydrophone, to the intensity, or response, 
on the acoustic axis. 

Doppler-Enabling System. A circuit which allows only 
echoes having doppler to actuate the torpedo control 
system. 

Echo Repeater. Artificial target, used in sonar calibration 
and training, which returns a synthetic echo by receiving, 
amplifying, and retransmitting an incident ping. 

FM Sonar. Scanning-type sonar using a continuous fre- 
quency-modulated transmission signal. 

Hydrofoil. A body so formed that its motion through the 
water produces desired forces upon its surfaces. 

Hydrophone. An underwater microphone. 

Magnetostriction Effect. Phenomenon exhibition by cer- 
tain metals, particularly nickel and its alloys, which change 
in length when magnetized, or which, when magnetized and 
then mechanically distorted, undergo a corresponding 
change in magnetization (Villari effect). 

ODN. Own doppler nullifier. 

Ping. Acoustic pulse signal projected by echo-ranging trans- 
ducer. 

Pip. Echo trace on indicator screen. 


Pitch. Angular deviation from the line of course of a pro- 
jectile taken in a vertical plane about its transverse axis. 

Projector. An underwater acoustic transmitter. 

Reverberation. Sound scattered diffusely back towards the 
source principally from the surface or bottom and from 
small scattering sources in the medium such as bubbles of 
air and suspended solid matter. 

SLC. Simultaneous lobe comparison. 

Spectrum Level. Sound pressure level in a 1-c band. 

Supersonic Frequencies. Range of frequencies higher than 
sonic, or *' ‘acoustic/ ’ frequencies. Sometimes referred to as 
ultrasonic to avoid confusion with growing use of supersonic 
to denote higher-than-sound velocities. 

Target Strength. Measure of reflecting power of target. 
Ratio, in decibels, of the target echo to the echo from a 6-ft 
diameter perfectly reflecting sphere at the same range and 
depth. 

Thyrite. A material whose impedance varies inversely as the 
cube of the current passing through it. 

Transducer. Any device for converting energy from one 
form to another (electrical, mechanical, or acoustical). In 
sonar, usually combines the functions of a hydrophone and a 
projector. 

TVG. Time- varied gain. 

USRL. Underwater Sound Reference Laboratories. 

Varistor. A dry rectifier with the characteristics of a non- 
linear resistance whose value decreases with increasing ap- 
plied voltage. 

X Cut. A cut in which the electrode faces of a piezoelectric 
crystal are perpendicular to an X-, or electrical, axis. 

Y Cut. A cut in which the electrode faces of a piezoelectric 
crystal are perpendicular to a Y-, or mechanical, axis. 

Yaw. Angular deviation from line of course of a projectile 
taken in a horizontal plane about its vertical axis. 

Z Cut. A cut in which the electrode faces of a piezoelectric 
crystal are perpendicular to a Z-, or optical, axis. 


161 
































































. 







BIBLIOGRAPHY 

Numbers such as Div. 6-913-M3 indicate that the document listed has been microfilmed and that its title appears in 
the microfilm index printed in a separate volume. For access to the index volume and to the microfilm, consult the 
Army or Navy agency listed on the reverse of the half-title page. 

PART I 


1. Torpedo Noise Tests, A Summary , H. J. Michael, Memo- 
randum 43-2450-HJM-AJ, BTL, Nov. 17, 1943. 

Div. 6-91 3-M3 

Summary of Underwater Torpedo Noise, R. J. Wylde, 
Report 4692, NOL, Dec. 15, 1943. Div. 6-913-M4 

2. Fundamentals of Hydro- and Aeromechanics Based on 
Lectures of Ludwig Prandtl, Oskar G. Tietjens, McGraw- 
Hill Book Co., Inc., New York, N. Y., and London, Eng., 
1934, p. 149. 

3. Observations of Cavitation on the Special Projectile ( Memo- 
randum ), Robert T. Knapp, Report ND-8.1, Hydraulic 
Machinery Laboratory, CIT, Nov. 24, 1942. 

Force and Cavitation Characteristics of the NACA J+I+12 
Hydrofoil, NDRC 6.1-sr207-1273, CIT, June 10, 1944. 

4. N otes on Hydraulic Noise, R. G. Folsom, E. D. Howe, and 

Morrough P. O’Brien, OSRD 949, NDRC C4-sr30-391, 
UCDWR, June 11, 1942. Div. 6-913-M2 

5. The Audio Frequency Phase System of Controls ( Part C), 
NDRC 6.1-sr785-962, BTL, Sec. 20. 

6. Measurements of the High Frequency Noise Produced by 
Cavitating Projectiles in the High Speed Water Tunnels , 
NDRC 6.1-sr207-924, CIT, Aug. 31, 1943. 

7. Self Noise Tests on Experimental Model of the Mark 31 
Mine , H. C. Montgomery, BTL, Jan. 21, 1944. 

Div. 6-913-M5 

8. Reduction of Torpedo Gear Noise, HUSL, Aug. 3, 1944. 

Div. 6-913-M6 

9. The Audio Frequency Phase System of Controls (Part B), 
NDRC 6.1-sr785-962, BTL, Sec. 12, p. 14. 

10. Ibid., Part D, Sec. 23, p. 1. 

11. The directivity index of a hydrophone is equal to the 
integral of the sensitivity over all directions divided by 47r 
times the maximum sensitivity and expressed in decibels. 
For details of this and other acoustic definitions, see A 
Practical Dictionary of Underwater Acoustical Devices , 
CUDWR-USRL, July 27, 1943. 

12. Calibration of Hydrophones in Mark 18 Nose Section, 
R. L. Hanson, BTL, Aug. 4, 1944, Fig. 7. 

Div. 6-923-M2 

13. Tests on Various Methods of Mounting Magnetostriction 
Hydrophones in Torpedo Heads, Robert C. McLoughlin 
and H. V. Knorr, HUSL, Aug. 1, 1944. Div. 6-911.1-MI 

14 Microphone Calibrations in Body 110 at Orlando, February 
1948, H. C. Montgomery, BTL, May 14, 1943. 

Div. 6-923- Ml 


15. Biweekly Report on Projects NO-149 and NO-157, Report 

XV, covering period from Nov. 3 to Nov. 16, 1943, 
NDRC 6.1-sr287-1169, HUSL, Nov. 19, 1943, Figures 
1A and IB. Div. 6-910-MI 

16. Biweekly Report on Projects NO-149 and NO-157, Report 

XIX, covering period from Dec. 29, 1943 to Jan 11, 1944, 
NDRC 6.1-sr287-1348, HUSL, Jan. 14, 1944. 

Div. 6-910-M2 

17. Biweekly Report on Projects NO-149 and NO-157, Report 

XX, covering period from Jan. 12 to Jan. 25, 1944, 
NDRC 6.1-sr287-1354, HUSL, Jan. 28, 1944. 

Div. 6-910-M3 

18. Biweekly Report on Projects NO-149 and NO-157, Report 
XXXIV, covering period from July 26 to Aug. 8, 1944, 
NDRC 6.1-sr287-1780, HUSL, Aug. 11, 1944. 

Div. 6-910-M4 

19. The Audio Frequency Phase System of Controls, NDRC 
6.1-sr785-962, BTL. 

20. Torpedo Studies, Summary Technical Report, NDRC 
Div. 6, Vol. 21. 

Dynamic Stability of Bombs and Projectiles, M. A. Biot, 
CIT, July 1, 1942, Chaps. I and II; Sept. 1, 1942, Chap. 
Ill; May 23, 1943, Chap. IV. 

21. Underwater Sound Reflecting Characteristics of Surface 
Ships (Memorandum), Case 23265-3, BTL, Oct. 6, 
1944. 

22. The Effectiveness of a 20-knot Acoustic Torpedo and of 

Possible Modifications Having Lower Speeds with or with- 
out an Automatic Speed Changing Mechanism, Conyers 
Herring and E. Ward Emery, NDRC 6.1-srll31-1882, 
CUDWR, Nov. 22, 1944. Div. 6-912.4-MI 

Comparison of Effectiveness of Acoustic Torpedoes with 
Non-Acoustic Torpedoes, C. L. Pekeris, OSRD 157, 
NDRC 6.1-srl 131-1 156, CUDWR, Feb. 16, 1944. 

Div. 6-900-M 1 

Countermeasures to the Acoustic Torpedo, ORG Memo- 
randum 42, Nov. 20, 1943. 

Measurements and Analysis of Sound Pressures of Tor- 
pedoes in Range 40 cps to 128 kc/s, A. B. Wood, OSRD 
WA-497-35, M/S Summary 6127/42, Mine Design De- 
partment, Hants, Eng., 1942. Div. 6-913-MI 

Project NO-181, an Echo-Ranging Antisubmarine Mine, 
OSRD 6606, NDRC 6.1-sr287-2089, HUSL, Jan. 1, 1946. 

Div. 6-920-M4 


163 


164 


BIBLIOGRAPHY 


PART II 


1. Preliminary Analysis of the Dynamics of Project NO-181 
Control ( Part I), Harvey A. Brooks, HUSL, June 2, 1944. 

Div. 6-921. 4-M2 

2. Analysis of the Dynamics of Project NO-181 Control ( Part 
77), Harvey A. Brooks, HUSL, Aug. 16, 1944. 

Div. 6-921. 4-M3 

3. Analysis of the Dynamics of Project NO-181 Control ( Part 
777), Harvey A. Brooks, HUSL, July 6, 1945. 

Div. 6-921. 4-M8 

4. Maximum Potentialities of an Echo-Ranging Torpedo, 
Harvey A. Brooks, HUSL, Nov. 1, 1944. Div. 6-920-MI 

5. General Electric Results on Echo Strength from a School- 
Type Submarine, Harvey A. Brooks, HUSL, Oct. 14, 1944. 

Div. 6-922-M2 

6. Theoretical Interpretation of General Electric Data on Sur- 

face Reverberation, Harvey A. Brooks, HUSL, Apr. 20, 
1945. Div. 6-922-M3 

7. Underwater Sound Reflecting Characteristics of Surface 
Ships, BTL, Oct. 6, 1944. 

8. Propeller Cavitation Theory and Experiments, Donald 

Ross, HUSL, May 28, 1945. Div. 6-934-M2 

9. The General Electric Torpedo Control System; Part I, In- 

formation Received from the Leeds and Northrup Group at 
the Florida Station, Vernon M. Albers, HUSL, Sept. 26, 
1945. Div. 6-922-M4 

10. The General Electric System of Torpedo Control; Part 77, 
Information Obtained from the Leeds and Northrup Group 
in Philadelphia, Vernon M. Albers, HUSL, Sept. 29, 1945. 

Div. 6-922-M5 

11. Miscellaneous Suggestions for Project NO-181-G, Harvey 

A. Brooks, HUSL, Jan. 12, 1945. Div. 6-920-M2 

12. High Power Pattern Measurements on Gamewell SPEP 

No. 2, Runs 1 through 8, Nicholas A. Abourezk, HUSL, 
Dec. 5, 1944. Div. 6-932-MI 

13. High Power Pattern Measurements on Gamewell SPEP 

No. 3, Runs 9 through 25, Nicholas A. Abourezk, HUSL, 
Mar. 1, 1945. Div. 6-932-M2 

14. Comparison of Harvard and General Electric Schemes, 
Harvey A. Brooks, HUSL, Oct. 6, 1944. Div. 6-922-MI 

15. Modification of Input Circuit for Project NO-181-G, 
Vernon M. Albers, HUSL, Jan. 18, 1945. 

Div. 6-921. 2-M6 

16. Behavior of the New Input Circuit for Project NO-181-G, 
Vernon M. Albers, HUSL, Mar. 12, 1945. 

Div. 6-921.2-M7 

1 7. Cross T alk and Sensitivity Requirements on Project N 0-1 8 1 , 
Harvey A. Brooks, HUSL, July 11, 1944. 

Div. 6-921. 2-M3 

18. The 1-kc Oscillator for Project NO-181-G, John G. King, 

HUSL, Mar. 21, 1945. Div. 6-921.3-M8 

19. New 60-kc Band-Pass Filters and Switching Tubes for 

Project NO-181-G, Vernon M. Albers, HUSL, Jan. 18, 

1945. Div. 6-921. 3-M7 

20. Calibration of Leeds and Northrup Acoustic Steering Units, 

B. D. Simmons, Report 4, U. S. Navy USRL, Orlando, 

Fla., July 30, 1945. Div. 6-932-M3 

21. Phase Sensitive Detector, Fred G. Gardner, HUSL, Apr. 1, 

1944. Div. 6-921. 3-M4 


22. Approximate Analysis of Automatic Volume Control Time 
Constants, Alfred W. Nolle, HUSL, Feb. 8, 1944. 

Div. 6-921. 3-M3 

23. The Automatic Volume Control Charge Time in Project 
NO-181, Harvey A. Brooks, HUSL, July 20, 1944. 

Div. 6-921. 3-M6 

24. Common Amplifier and Automatic Volume Control System 

for Project NO-181, Nicholas A. Abourezk, HUSL, Jan. 
20, 1944. Div. 6-921.3-M2 

25. Diode-Type Electronic Switch for Use as a Doppler Gate 

in Project NO-181, Nicholas A. Abourezk, HUSL, Nov. 
24, 1943. Div. 6-921.3-MI 

26. Comments on Leeds and Northrup Transducers , Harvey A. 

Brooks, HUSL, Nov. 19, 1945. Div. 6-932-M4 

27. Bearing Deviation Indicator Input Circuits Used in Project 
NO-181, Harvey A. Brooks, HUSL, May 5, 1944. 

Div. 6-921. 2-MI 

28. The Use of Gyro Rate of Turn Control in Project NO-181, 
Harvey A. Brooks, HUSL, May 4, 1945. 

Div. 6-921.4-M7 

29. Modification of Bearing Deviation Indicator Input Circuit 

for Project NO-181, Harvey A. Brooks, HUSL, June 3, 
1944. Div. 6-921. 2-M2 

30. A pplication of A utomatic Volume Control to Project N O-l 81 
Input Amplifier, John G. King, HUSL, May 22, 1944. 

Div. 6-921.3-M5 

31. Phase Sensitive Detectors in Project NO-181, A. Nelson 

Butz, Jr., HUSL, Nov. 15, 1944. Div. 6-921. 2-M4 

32. Addendum to Phase Sensitive Detectors in Project NO-181, 
A. Nelson Butz, Jr., HUSL, Nov. 16, 1944. 

Div. 6-921.2-M5 

33. The Project NO-181-G Glide Angle Control, Frank S. 
Replogle, Jr., and Joseph M. Bringman, HUSL', Jan. 12, 

1944. Div. 6-921. 4-M5 

34. Work Done on Glide Angle Control , Supplementary Report 
for Week Ending Thursday, February 22, 1945, Frank S. 
Replogle, Jr., and Joseph M. Bringman, HUSL, Feb. 23, 

1945. Div. 6-921.4-M6 

35. Quantitative Specifications on Electronic Glide Angle Con- 

trol, John G. King and Harvey A. Brooks, HUSL, Nov. 
24, 1944. Div. 6-921.4-M4 

36. Dynamical Behavior of the Project 61 Body on the Florida 

Runs, February to March 1945, Frank S. Replogle, Jr., 
HUSL, Aug. 24, 1945. Div. 6-921.4-M9 

37. The Amplitude Gate and Doppler Gate System, Vernon M. 

Albers, HUSL, Dec. 17, 1945. Div. 6-921.3-M9 

38. The Effect of Noise on Own Doppler Nullifier Frequency 
Setting , Frank S. Replogle, Jr., HUSL, May 17, 1945. 

Div. 6-921. 1-M5 

39. Frequency Spread in Reverberations, Response of Dis- 
criminator, Malcolm H. Hebb, HUSL, Oct. 2, 1944. 

Div. 6-921. 1-M2 

40. The Theory of Discriminator Response to Reverberation 

( First Installment), Harvey A. Brooks, HUSL, May 21, 
1945. Div. 6-921. 1-M6 

41. Frequency Fluctuations in Reverberation at Spy Pond, 

Harvey A. Brooks and Nicholas A. Abourezk, HUSL, 
Mar. 27, 1945. Div. 6-921. 1-M4 


BIBLIOGRAPHY 


165 


42. The Theory of Discriminator Response to Reverberation 

( Second Installment ), Harvey A. Brooks, HUSL, June 4, 
1945. Div. 6-921. 1-M 7 

43. Theory of Own Doppler Nullifier Correction Rate , Harvey 
A. Brooks, HUSL, July 18, 1944. Div. 6-921. 1-M 1 

44. Further Analysis of First Tests on Reverberation Spread, 

Harvey A. Brooks and Nicholas A. Abourezk, HUSL, 
Jan. 18, 1945. Div. 6-921. 1-M3 

45. Relay Amplifiers for the Project 4-G2, Fish, Vernon M. 

Albers, HUSL, Dec. 17, 1945. Div. 6-921.5-MI 

46. The Control Relay System in the Panel of the Project 4-G2, 
Fish, Vernon M. Albers, HUSL, Jan. 23, 1946. 

Div. 6-921.5-M2 

47. The Afterbody Circuits for the Project 4-G2 Torpedo, Vernon 
M. Albers, HUSL, Jan. 23, 1946. Div. 6-921. 5-M3 

48. Application of an Echo Control System in the Mark 14 

Torpedo, NDRC 6.1-srl097-2342, OEMsr-1097, Project 
NO-157-B, BTL, Aug. 31, 1945. Div. 6-923-M5 

49. Circuit Design Considerations for Project NO-1 57 -C , 

BTL. Div. 6-912. 1-M4 

50. Additional Notes on BTL Meeting, of February 6, 1945, 
Harvey A. Brooks, HUSL, Feb. 17, 1945. 

Div. 6-920-M3 

51. Progress Report on Project NO-157-B, NDRC 6.1-srl097- 
1330, OEMsr-1097, BTL, Mar. 1, 1945. Div. 6-923-M4 

52. Noise Data Taken on Spy Pond Captive Mark 18 Torpedo, 

Lyman N. Miller, Roland Mueser, and P. M. Kendig, 
HUSL, May 8, 1944. Div. 6-912.1-MI 

53. Noise Data on Mark 18 Torpedo, Donald Ross, HUSL, 

June 2, 1945. Div. 6-912. 1-M3 

54. A Report on Bell Telephone Laboratories Trip by Albers 

and Graber, Vernon M. Albers and Ray Graber, HUSL, 
Oct. 30, 1945. Div. 6-923-M6 


55. A Report on Trip to Bell Telephone Laboratories at Murray 
Hill, New Jersey; Part I, Transducer, Nicholas A. 
Abourezk and Robert H. Lefkovich, HUSL, Nov. 1, 1945. 

Div. 6-923-M7 

56. A Report on Trip to Bell Laboratories at Murray Hill, New 

Jersey; Part II, Mechanical Gear, Nicholas A. Abourezk, 
HUSL, Nov. 8, 1945. Div. 6-923-M8 

57. Preliminary Report on Geier; Part I, Electronic System, 
Vernon M. Albers, HUSL, June 17, 1945. 

Div. 6-925-M 1 

58. Second Report on Geier, Vernon M. Albers, HUSL, July 5, 

1945. Div. 6-925-M 3 

59. Probable Tactics of Geier, Harvey A. Brooks, HUSL, June 

18, 1945. Div. 6-925-M 2 

60. The Geier Torpedo Control and How It Compares with the 

Harvard Project NO-181 System, Vernon M. Albers, 
HUSL, Aug. 1, 1945. Div. 6-925-M4 

61. Measuring Self-Noise of Geier, John J. Iffland, HUSL, 

Oct. 26, 1945. Div. 6-925-M6 

62. Conference with Lieutenant Colonel Bree of the Luftwaffe, 
Harvey A. Brooks, HUSL, Dec. 4, 1945. Div. 6-925-M7 

63. British Admiralty Delegation Report, British Homing 
Torpedoes, Sept. 12, 1945. 

64. Information on British Torpedo Projects, Harvey A. 

Brooks, HUSL, Dec. 6, 1944. Div. 6-924-M3 

65. Bowler Torpedo Control Development, Rodney F. Simons, 
OSRD WA-1384-4A, Torpedo Technical Memorandum 1, 
OSRD Liaison Office, Great Britain, Dec. 23, 1943. 

Div. 6-924-M 1 

66. Bowler, Rodney F. Simons, OSRD WA-1470-6A, Torpedo 

Technical Memorandum 2, OSRD Liaison Office, Great 
Britain, Jan. 14, 1944. Div. 6-924-M2 


Supplementary References 

LISTENING TORPEDOES 


Self-Produced Noise from a MarkXXIV Mine, K. C. Morrical, 
Ray S. Alford, and others, NDRC 6.1-sr287 and sr785-720, 
HUSL and BTL, Jan. 9, 1943. Div. 6-911.3-MI 

Discussion of the Stability of the Mark XXI V Mine in Terms 
of Electrical Feedback Theory, J. C. Lozier and A. C. Dickie- 
son, Report 43-3510-ACD-JCL-TP, BTL, Mar 19, 1943. 

Div. 6-911.3-M2 

The Dynamical Performance of the Rudder-Rudder Motor 
System Used on the Mark 24 Mine, R. Clark Jones, BTL, 
Mar. 25, 1943. Div. 6-911.3-M3 

The Audio Frequency Phase System of Control for the Mark 
XXIV Mine: Part A, Description of Apparatus; Part B, 
Measurements and Tests; Part C, Theory of Operation; Part 
D, Supplementary Memoranda, OEMsr-346 and OEMsr-785, 
OSRD 1992, NDRC 6.1-sr785-962, BTL, July 1943. 

Div. 6-911.3-M5 

Discussion of the Tracking Performance of the Mark XXIV 
Mine in Terms of Electrical Feedback Theory, J. C. Lozier, 
Report 43-3510-JCL-HS, BTL, Oct. 26, 1943. 

Div. 6-911.3-M6 

Trajectory Calculations for the Project NO-1 57 -B Torpedo, R. 
Clark Jones, BTL, Aug. 25, 1944. Div. 6-923-M3 


Studies of Project 0, R. L. Peek, Jr., OEMsr-1097, NDRC 

6.1- srl097-2340, BTL, Aug. 14, 1945. Div. 6-925-M5 
Torpedo Mark 27, OEMsr-1294, NDRC 6.1-srl294-2338, 

BTL, Aug. 17, 1945. Div. 6-912.3-MI 

The Mark 28 Torpedo, History, Principles of Operation of the 
Acoustic Control Circuit and Field Performance, NDRC 

6. 1- sr 1097-2339, BTL, Aug. 20, 1945. Div. 6-912.4-M2 
Final Technical Report of Contract OEMsr-1097 , OEMsr-1097, 

NDRC 6.1-sr287-2090, HUSL, Jan. 1, 19 v 46. Div. 6-910-M5 
Memorandum on Torpedo Steering, W. V. Houston, CUDWR 
Special Studies Group, Feb. 14, 1944. 

A Torpedo Survey on Project N-121, NDRC 6.1-srll31-1892, 
CUDWR, Dec. 22, 1945. Div. 6-900-M3 

Self -Noise Measurements on Mark 24 Mine, NDRC 6.1-sr287- 
900, HUSL, June 28, 1943. Div. 6-911.3-M4 

Mark 24 Mine Performance Tests, NDRC 6.1-sr287-1176, 
HUSL, Dec. 8, 1943. Div. 6-911.3-M7 

Torpedo Depth Steering, NDRC 6.1-sr287-1457, HUSL, Apr. 

8, 1944. Div. 6-921.4-MI 

Propeller Program at the Harvard Underwater Sound Labora- 
tory, OEMsr-287, NDRC 6.1-sr287-2170, HUSL, Feb. 15, 
1945. Div. 6-934-MI 


166 


BIBLIOGRAPHY 


Torpedo Instrumentation , A Summary of Torpedo Instruments 
Constructed by HUSL , NDRC 6.1-sr287-2174, HUSL, Mar. 
5, 1945. Div. 6-900-M2 

An Acoustically Controlled Electric. Torpedo , An Adaptation of 
the Mark 18 Electric Torpedo to Acoustic Control at 20 and 30 
Knots ( Diagrams Included ), NDRC 6.1-sr287-2177, HUSL, 
Mar. 10, 1945. Div. 6-912. 1-M2 

Noise Measurements on a Mark 21 Torpedo , NDRC 6.1-sr287- 
2178, HUSL, Mar. 15, 1945. Div. 6-911.2-M2 

Acoustic Locating System, OEMsr-287, OSRD 6613, NDRC 
6.1-sr287-2056, HUSL, Apr. 5, 1945. Div. 6-935-MI 

The Projects NO-149-F and NO-157-F Test Equipment, 
NDRC 6.1-sr287-2186, HUSL, May 15, 1945. 

Div. 6-931-MI 

Beeper Transmitter, Model 3, NDRC 6.1-sr287-2189, HUSL, 
June 1, 1945. Div. 6-935-M2 

Miscellaneous Recording Equipment for Controlled Torpedoes, 
OSRD 6619, NDRC 6.1-sr287-2080, HUSL, Dec. 1, 1945. 

Div. 6-931-M2 

Machinery Noise of the Electrical Torpedo, OEMsr-287, 
OSRD 6436, NDRC 6.1-sr287-2053, HUSL, Dec. 15, 1945. 

Div. 6-91 3-M7 

Foreign Ordnance, Project G, OEMsr-287, OSRD 6439, 
NDRC 6.1-sr287-2077, HUSL, Jan. 1, 1946. 

Div. 6-925-M8 

Miscellaneous Electronic Equipment for Controlled Torpedoes, 
OEMsr-287, OSRD 6548, NDRC 6.1-sr287-2081, HUSL, 
Jan. 1, 1946. Div. 6-931-M3 


Propeller Design Studies, OEMsr-287, NDRC 6.1-sr287-2093, 
HUSL, Jan. 1, 1946. Div. 6-934-M3 

An Air-Launched Acoustic Torpedo, Project NO-149, OSRD 
6620, NDRC 6.1-sr287-2090, HUSL, Jan. 1, 1946. 

Div. 6-911-MI 

Project NO-157, A Submarine-Launched Acoustic Torpedo , 
OSRD 6626, NDRC 6.1-sr287-2091, HUSL, Jan. 1, 1946. 

Div. 6-912-MI 

Torpedo Control and Protective Devices, OEMsr-287, OSRD 
6632, NDRC 6.1-sr287-2092, HUSL, Jan. 1, 1946. 

Div. 6-921. 4-M 10 

An Air-Launched Acoustic Antisubmarine Mine, Project NO-94, 
Fido, OSRD 6648, NDRC 6.1-sr287-2078, HUSL, Jan. 1, 
1946. Div. 6-911.3-M8 

Project NO-1 49, Mark 21 Torpedo ( Final Report under Con- 
tract OEMsr-1051), OSRD 5015, NDRC 6.1-srl051-2121, 
Westinghouse Electric and Manufacturing Co., Feb. 28, 
1945. Div. 6-911.2-MI 

Mark 22 Torpedo, Final Report of Project NO-157, OEMsr- 
1053, NDRC 6.1-srl053-2125, Westinghouse Electric and 
Manufacturing Co., May 23, 1945. Div. 6-912.2-MI 

Counter-Rotating Motor for Torpedo Drive, Gerhard Mauric, 
OSRD 6659, NDRC 6.1-srl370-2397, Electrical Engineering 
and Manufacturing Corp., Apr. 15, 1946. Div. 6-933-MI 
Torpedo Retrieving Gear , OSRD 6543, NDRC 6.1-sr287-2094. 

HUSL, Dec. 15, 1945. Div. 6-935-M3 

An Echo Control System for the Mark 13 Torpedo Project 
NO-149-C, OSRD 6069, NDRC 6. 1-sr 1097-2341, BTL, 
Aug. 27, 1945. Div. 6-911.1-M2 


SKCRKT 


CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS 


Contract Number Name and Address of Contractor 

Subject 

OEMsr-20 

The Trustees of Columbia University 
in the City of New York 

New York, N. Y. 

Studies and experimental investigations in connection 
with and for the development of equipment and 
methods pertaining to submarine warfare. 

OEMsr-1131 

The Trustees of Columbia University 
in the City of New York 

New York, N. Y. 

Conduct studies and investigations in connection with 
the evaluation of the applicability of data, methods, 
devices, and systems pertaining to submarine and 
subsurface warfare. 

OEMsr-287 

President and Fellows of Harvard College 
Cambridge, Mass. 

Studies and experimental investigations in connection 
with (1) the development of equipment and devices 
relating to subsurface warfare. 

OEMsr-323 

General Electric Company 

Schenectady, New York 

Studies, experimental investigations, and development 
work in connection with submarine and subsurface 
warfare. 

OEMsr-785 

Western Electric Company, Inc. 

New York, N. Y. 

Studies and experimental investigations in connection 
with Project 61. 

OEMsr-1097 

Western Electric Company, Inc. 

New York, N. Y. 

Conduct studies and experimental investigations in 
connection with the development, design and con- 
struction of preproduction models of the acoustic 
and electronic arrangements required for Projects 

NO-149 and NO-157, and such other development, 
design, and construction work in this connection that 
may be required. 

OEMsr-1294 

Western Electric Company, Inc. 

New York, N. Y. 

Conduct studies and experimental investigations in 
connection with production designs for the extension 
of Navy Project NO-94. 

OEMsr-1051 

Westinghouse Electric Corporation 

Sharon, Pa. 

Studies and experimental investigations in connection 
with testing Mark 18 samples, and the design, de- 
velopment, construction, and testing of launching 
samples of an aerial torpedo, acoustically controlled 
and electrically propelled. 

OEMsr-1053 

Westinghouse Electric Corporation 

Sharon, Pa. 

Studies and experimental investigations in connection 
with testing Mark 19 samples and the design, de- 
velopment, construction and testing of two hand-made 
samples of an acoustically controlled, electrically 
propelled submarine torpedo. 

OEMsr-1370 

Electrical Engineering and Mfg. Corp. 

Los Angeles, Calif. 

Conduct studies and experimental investigations in 
connection with electric motor development. 

OEMsr-1419 

Leeds and Northrup Company 
Philadelphia, Pa. 

Conduct engineering studies and design work on a con 
trolled mine, including some model shop work; en- 
gineering design and construction of a small number 
of the preproduction models. 


167 


SERVICE PROJECT NUMBERS 


The projects listed below were transmitted to the Executive Secretary, 
National Defense Research Committee [NDRC], from the War or 
Navy Department through either the War Department Liaison Officer 
for NDRC or the Office of Research and Inventions (formerly the Co- 
ordinator of Research and Development), Navy Department. 


Service 

Project 

Number 


Subject 


NO-94 


Mark — Mine 

Acoustic control for torpedoes 

Acoustically directed 21-inch torpedo for submarines 
Echo ranging control 


NO-149 


NO-157 


NO-181 


INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. 

For access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


Acoustic homing torpedoes; see Echo- 
ranging torpedo control systems, 
Mines, acoustic homing, Torpe- 
does, acoustic homing 
Acoustic pressure in terms of electric 
power, 64 

Acoustic range of homing torpedoes; 
see also Homing range of echo- 
ranging torpedoes 
echo-ranging formulas, 64-66 
limitations, 6, 51, 56, 159 
Aircraft launched torpedoes, control 
systems; British Bowler system, 
154 

British Dealer system, 104 
GE N0181 system, 76 
Geier 2 system, 143-148 
general design considerations, 72, 
157 

Amplifier balancing; pilot channel sys- 
tem, 47 

switching method, 46 
Amplitude gates for operation switch- 
ing; in HUSL N0181 system, 
85, 97 

in ORL project 4 system, 109, 111 
operation, 47 

Antisubmarine torpedo control sys- 
tems; British Dealer system, 104 
GE N0181 system, 76-84 
general design considerations, 66, 
72-75 

HUSL N0181 system, 85-103 
Antisurface-ship torpedo control sys- 
tems; British Bowler system, 
52, 154 

British Trumper System, 149-153 
BTL 157B and 157C systems, 120- 
137 

Geier 1; 138-148 
Geier 2; 143-148 

general design considerations, 66, 72- 
75 

ORL project 4 system, 105-119 
Atlas Werke Munich, 138 
Attenuation of sound in sea, frequency 
dependence, 9, 51 

Automatic steering control; see Steer- 
ing control 

Automatic volume control; in HUSL 
system, 98 

use in echo-ranging receivers, 69 
with TVG in ORL system, 109 
Axis of transducer, def., 64 
Azimuth steering; see Steering con- 
trol 


Balancing methods for dual amplifiers, 
46 

Balancing torpedo propeller torque, 
18 

Band width formula, echo-ranging pro - 
jector, 51 

Beam pattern, echo-ranging projector, 
50 

Bell Telephone Laboratories, 157B and 
157C systems, 120-137, 157 
Bernoulli’s theorem, 11, 12 
Blanking circuit GE N0181 system, 83 
Bowler steering method for echo-rang- 
ing torpedoes, 52, 56, 154, 157 
British Bowler system, 52, 56, 154, 157 
British Dealer system, 104 
British torpedoes, engine noise, 20 
British Trumper system, 149-153, 157 

Camoperated switches, echo ranging 
time base, 70, 78, 106 
Cavitation, factors effecting; around 
moving sphere, 11 
around torpedoes, 10-17 
due to diminishing cross section of a 
streamline, 10 
vortex formulas, 10 
Cavitation coefficient, 12 

values for various shapes, 12, 14 
Cavitation noise, 14, 25, 27 
function of torpedo speed, 9 
hydrophone discrimination, 23 
identification, 16 
speed of onset, 9 
Cavitation parameter, 12, 13, 25 
Circuit arrangements in torpedo after- 
body, 117 

Circuits, electronic; amplitude gates 
for switching, 47, 85, 97, 109, 111 
AVC, 69, 98, 109 
blanking circuit, 83 
differential operated gate, 48 
limiter circuit, 128 
preference circuit, 130 
pulse shaping circuit, 108 
range reduction circuit, 153 
threshold circuit, 126 
transmitter circuit, 76, 124, 141, 149, 
154 

transmitter receiver circuit, 125 
trigger circuit, 132 
TVG, 55, 69, 80, 109, 127 
“Circulation” of a vortex, def., 10 
Climb angle limiter for depth steering 
control, 49 

Coefficient of cavitation, 12 


Coefficients of hydrodynamic forces, 
def., 41 

Collision course steering, 152 
Commutation method for amplifier bal- 
ance, 46 

Comparison bridge, GE N0181 system, 
81 

Control systems; see Steering con- 
trol for echo-ranging torpedoes, 
Steering control for listening 
torpedoes 

Counter rotating field and armature 
type torpedo motor, 20 
Counter rotating torpedo propellers, 
20 

Countermeasures for homing torpe- 
does, 68, 157, 158 
Cross force coefficient, 41 
Crystal transducers for echo-ranging 
torpedoes, 121, 149 
Cuton-cutoff steering, 74, 82, 138-148 
evaluation, 69, 156 

Damping factor, hydrodynamic, 37-45 
Damping moment coefficients, 41 
Dealer system, British, 104 
Decibels spectrum level, 7 
Definitions, echo-ranging terms and 
units, 63-69 

Depth steering control; analysis, 39 
British Dealer system, 104 
ExFER42 mine, 57 
GE N0181 system, 82 
general design considerations, 49, 
72-75 

HUSL N0181 system, 102 
restricted to 250 yd range, 118 
Detectors, phase sensitive; in British 
Trumper system, 152 
in BTL 157C system, 129-130 
in HUSL N0181 system, 93, 101 
in ORL project 4 system, 111 
Differential operated gate, 48 
Direction finding systems for echo- 
ranging torpedoes; see Steering 
control for echo-ranging torpe- 
does 

Direction finding system for listening 
torpedo, 31-36 

Directivity index, transducer; defini- 
tion, 63 

formulas, 23, 64 
of HUSL transducer, 76 
Directivity patterns, hydrophones, 23 
Doppler effect used to identify echoes, 
54 




169 


170 


INDEX 


Doppler-enabling receiver, ORL proj- 
ect 4 system, 112 

Doppler-enabling system; advantages, 
156, 158 

CRO patterns, 100 
HUSL N0181 system, 85, 94, 100 
Drag force coefficient, 41 

Echo identification; by use of doppler 
effect, 54 

by use of long signal, 54 
by use of modulated signal, 54 
by use of short signal, 53 
by use of time variable gain, 55 
Echo level, factors effecting, 52 
Echo ranging torpedo control systems; 
advantages, 68-69, 73 
British Bowler, 52, 56, 154, 157 
British Dealer system, 104 
British Trumper, 149-153, 157 
BTL 157B and 157C, 120-137, 157 
disadvantages, 68 
evaluation, 156-160 
GE N0181 system, 76-84, 156 
Geier 1 and 2; 138-148, 157 
general design considerations, 50-57, 
63-75 

HUSL N0181; 85-103, 156 
ORL project 4; 105-119 
Echo relay, ORL project 4 system, 115 
Echo-to -reverberation ratio, 75 
Electronic time base for echo ranging 
systems, 70 

Ellipsoids, cavitation around, 13 
Enabling relay, ORL project 4 system, 
115 

Equations of motion for homing torpe- 
does, 37-45, 74 

Equivalent sphere formula, target 
strength, 65 
Ex20F torpedo, 18, 49 
ExF42 mine; amplifier balance, 46 
cavitation noise, 16, 24 
hydrophone directional patterns, 23 
machinery noise, 18, 21 
rudder operation, 48 
search mechanism, 33 
ExFER42 mine; amplifier balance, 46 
depth control, 57 
echo identification, 54 
scanning system, 50 
self-noise level, 52 
steering and searching system, 57 
ExFF3 torpedo, 3, 18 
PNplosive charge, used for rudder con- 
trol, 155 

ExS13 mine, cavitation noise, 16 
ExS29 torpedo, 20 

Fairprene for acoustic isolation, 18 
Fin cavitation, 13 


Force equations, hydrodynamic, 41 
Frequency characteristics of machin- 
ery noise, 21 

Frequency dependence, torpedo self- 
noise, 9 

Frequency distribution of echo energy, 
51 

G7e submarine torpedo, 139 
Gas flow noise in pneumatic lines, 21 
Gates in torpedo steering circuits; dif- 
ferential operated, 48 
doppler operated, 85 
level operated, 47, 85, 95, 108 
Gear noise in torpedoes, 18 
Geier 1 torpedo control system, 138- 
148 

differences, aircraft and submarine 
types, 143 

evaluation of performance, 143, 157 
Geier 2 torpedo control system, 143- 
146 

General Electric NO 181 system, 76-84, 
156 

German torpedo control systems, 138- 
148, 158 

Glide angle control, ORL project 4 sys- 
tem, 118 

Gyro course correction steering 
method, 57 

Gyro lock-off, ORL project 4 system, 
118 

Harvard Underwater Sound Labora- 
tory N0181 system, 85-103, 156 
High speed water tunnel (CIT), 12, 
14 

Homing range of echo-ranging torpe- 
does; as function of self noise, 65 
formulas, 64-66 
limitations, 4, 52, 56, 159 
maximum effective range, 56 
of BTL 157C system, 120 
of GE N0181 system, 76, 83 
of Geier control system, 138, 146 
Homing torpedoes; disadvantages, 4 
general design considerations, 1-2, 
58 

Hydrodynamics, torpedo, 37-45 
coefficients, 41 

constants for Mark 13 torpedo, 
43 

equations of force, 41 
Hydrophone mounting, 154 
Hydrophones, torpedo; see also Trans- 
ducers, echo-ranging 
British Trumper system, 150 
directivity patterns, 23, 65 
effect of unbalance, 34 
isolation, 23, 24 
noise discrimination, 23-30 


research recommendations, 59 
terminology and units, 63-66 

Identification of echoes; using doppler 
effect, 54 

using long signal, 54 
using modulated signal, 54 
using short signal, 53 
using time variable gain, 55 
Impedance of HUSL transducer ele- 
ments, 77, 86 

Intensity units, acoustic, 63 
Irrotational motion, def. 10 

Lag line angle formula, 89 
Laminated stack magnetostrictive 
transducer elements, 76, 86, 140 
Level-operated switching gate, 47, 85- 
97, 108-109 

Limiter circuit, BTL 157C system, 
128 

Listening torpedo control systems, 31- 
36 

countermeasures, 68 
Long signal method for echo identifica- 
tion, 54 

Machinery noise in torpedoes, 18-22 
dependence on torpedo speed, 26 
frequency spectrum, 21 
hydrophone discrimination 24, 30 
Magnetic homing, 5 
Magnetostrictive transducer elements, 
76, 85, 140 
Mark 9 torpedo, 149 
Mark 13 torpedo; fin cavitation, 13 
self noise, 8 

Mark 13 torpedo with shroud ring, 
hydrodynamic constants, 43 
Mark 14 torpedo, 57, 120 
Mark 18 torpedo, 8, 18, 28, 120 
Mark 20 torpedo, 159 
Mark 26 torpedo, 20 
Maximum homing range; function of 
self noise, 66 

limitations, 4, 52, 56, 159 
of BTL 157C system, 121 
of GE N0181 system, 76 
of Geier control system, 139, 148 
Minerva Radio, Vienna, 138 
Mines, acoustic homing; ExF42 mine, 
3, 16-22, 33, 46-48 
ExFER42 mine, 46-57 
ExS13 mine, 16 

Mines, wake following; see Wake-fol- 
lowing torpedoes and mines 
Minor lobes, defined, 64 
Moment coefficient, 41 
Momentum of torpedo, equation, 41 
Motor, counter-rotating field and ar- 
mature type, 20 



INDEX 


171 


Motor noise in torpedoes, 21 
Motor operated rudders, 48 
Multivibrator operated time-base, 91, 
123 

Nickel lamination transducers, 76, 86, 
140 

X0181 system, GE, 76-84 
N0181 system, HUSL, 85-103 
Noise discrimination in hydrophones; 
against cavitation noise, 23 
against machinery noise, 24 
against water background noise, 23 
Noise discriminator; Geier 2 system, 
144 

Noise reference level, defined, 63 
Noise, torpedo, 7-30 
cavitation noise, 10-17 
machinery noise, 18-22 
self noise, 7-9 
total noise, 25-30 
water noise, 7, 23 

Ogives, cavitation around, 13 
157B control system, BTL, 120-137 
157C control system, BTL, 120-137 
On-off steering, 72, 74, 82, 138-148 
evaluation, 69, 156 

Ordnance Research Laboratory project 
4 system, 105-119 

Oscillating motion of homing torpedo; 
conditions for, 40-45 
tendency in BTL 157C system, 
135 

Own-doppler correction, HUSL sys- 
tem, 95, 100 

Pendel-Rose stabilized transducer as- 
sembly, 147 

Pendulum type limiter for depth-steer- 
ing control, 49 

Phase difference system for direction 
finding, 36 

Phase sensitive detectors; in British 
Trumper system, 152 
in BTL 157C system, 129-130 
in HUSL N0181 system, 93, 101 
in ORL project 4 system, 111 
Piezo-electric crystal transducer, 121, 
149 

Pilot channel system for amplifier 
balancing, 47 

Power absorbed by hydrophone, band 
width effect, 63 

Power limitations of torpedo trans- 
ducers, 50 

Preference circuit, BTL 157C system, 
130 

Preferred side steering, 121, 130, 139, 
145 

evaluation 146-148, 158 


Pressure in sound field; as a function 
of electric power, 64 
units defined, 63 

Project 4 system, ORL, 105-119 
Projector, echo-ranging; see Transduc- 
ers, echo-ranging 
Propeller cavitation, 9, 12 
caused by blade defects, 15 
caused by vortexes, 11 
effect of propeller tip speed, 14* 25 
hydrophone discrimination against, 
24 

Propeller modulation of shipnoise, 7 
Propeller thrust coefficient, 41 
Propeller vibration noise, 22 
Pulse shape, Geier system, 143 
Pulse shaping circuits; ORL project 4 
system, 108 

Quartz sandwich type transducers, 149 

Radiation resistance; definition, 64 
value for water, 64 
Radio controlled torpedoes, 5 
Range, homing, of echo-ranging tor- 
pedos; formulas, 64-66 
function of self noise, 65 
limitations, 5, 52, 56, 160 
of BTL 157C system, 120 
of GE NO 181 system, 76, 83 
of Geier control system, 138, 146 
Range of target, effect on echo strength, 
52 

Range reduction circuit, GE NO 181 
system, 83 

Receiver amplifiers, BTL 157C system, 
125 

Receivers, doppler-enabling; HUSL 
NO 181 system, 93 
ORL project 4 system, 112 
Receivers, echo-ranging; British Dealer 
system, 104 
GE NO 181 system, 80 
Geier 1 system, 141 
general design considerations, 70 
Receivers, steering; British Trumper 
system, 150 

HUSL N0181 system, 92, 101 
ORL project 4 system, 108 
Recommendations for further research; 
depth steering, 49 
hydrophone studies, 59 
reduction of hydrophone water 
noise, 21 

self noise studies, 58 
tactical use of homing torpedoes, 
58-59 

variable speed torpedo, 6 
Relaxation time of torpedo body, 74 
Relay control circuits; British Trumper 
system, 152 


ORL project 4 system, 114-116 
Relays in echo-ranging control sys- 
tems, 71, 114-116, 135-137, 142 
see also Gates 

Reverberation discrimination, 53-55 
Rudder control, torpedo; see also Steer- 
ing control 

automatic reversal, 116 
equation of motion, 74 
motor-operated, 48 
solenoid operated, 49 
using explosive charge, 155 
Rudder torque constant, torpedo, 37- 
45 

Salvo firing with echo -ranging torpe- 
does, 68 

Scanning system, echo-ranging pro- 
jector, 50 

Search mechanisms ; British dealer sys- 
tem, 104 

depth behavior, 72 
ExF42 mine, 33 

ExFER42 mine combined steering 
and search system, 57 
HUSL N0181 system, 102 
ORL project 4 system, 105 
Self noise, torpedo, 6, 7-30, 65 
dependence on depth, 28, 159 
dependence on speed, 8, 28 
experimental analysis, 16 
frequency dependence, 9 
Mark 18 torpedo, ORL system, 
105 

measurement by external hydro- 
phone, 8, 27 

measurement by self hydrophones, 
27 

research recommendations, 58 
Selsyn, proposed use in control systems, 
159 

Sensitivity of hydrophone, definition, 
63-64 

Servo systems; see Steering control 
Short signal method for echo identifica- 
tion, 53 

Signal generator limitations, torpedo, 
70 

Signal level requirements in torpedo 
echo-ranging, 50-52 
Solenoid-operated rudders, 49 
Sound pressure, units defined, 63 
Spectrum level of acoustic energy, 
formula, 51 

Sphere, cavitation about, 11 
Sphere, equivalent to target, formula, 
65 

Spin tendency of torpedoes, 49 
Stability, torpedo, 37-45, 49 
Steering control, mathematical analy- 
sis, 37-45 




172 


INDEX 


Steering control for echo ranging tor- 
pedoes; British Bowler system, 
56, 154 

British Dealer system, 104 
British Trumper system, 152 
BTL 157C system, 132 
circuit arrangement in torpedo after- 
body, 117 

collision course steering, 152 
ExFER42 mine method, 57 
GE NO 181 sys em, 82 
Geier 1 system, 138 
Geier 2 system, 144 
general design considerations, 49-57, 
63-75, 160 

gyro course correction method, 57 
HUSL N0181 system, 102-103 
on-off steering, 69, 72, 82, 138-148 
preferred side steering, 121, 130, 144- 
148, 158 
selsyns, 159 

‘‘snaky” course, 73, 105 
symmetrical steering, 144 
vertical steering, 39, 49, 72-75 
Steering control for listening torpedoes, 
intensity difference method, 31- 
36 

phase difference method, 36 
Stiffness of control, torpedo steering 
mechanism, 37-45 

Submarine launched torpedoes, con- 
trol systems; British Trumper 
system, 149 
BTL 157C system, 120 
Geier 1 system, 138-148 
Surface ship launched torpedoes, con- 
trol systems; general design 
considerations, 66, 72 
Switching circuit, transmitter receiver, 
125 

Switching method for amplifier balanc- 
ing, 46 

Symmetrical steering, 139, 146-148 

Tactical use of homing torpedoes, rec- 
ommendations, 58-59 
Target location system for listening 
torpedo, 31-36 

Target noise, effect on useful range, 51 
Target range; see Homing range of echo 
ranging torpedoes 
Target strength formulas, 51, 65 


Threshold circuit, BTL 157C system, 
126 

Time base for echo-ranging systems; 
British Dealer system, 104 
BTL 157C system, 122 
cam-operated switches, 70, 78, 106 
electronic, 70 
GE N0181 system, 78 
general design considerations, 70 
HUSL N0181 system, 91 
multivibrator operated, 91, 123 
ORL project 4 system, 106, 117 
Time lag in acoustic control systems, 
analysis, 39-45, 48 

Time varied gain circuits, in BTL 157C 
system, 127 

in GE NO 181 system, 80 
in ORL project 4 system, 109 
use in echo-ranging receivers, 55, 
69 

Torpedoes, acoustic homing; see also 
Mines, acoustic homing 
Ex 20F torpedo, 18, 49 
Ex S29 torpedo, 20 
57C submarine torpedo, 139 
limitations, 6 
Mark 9 torpedo, 149 
Mark 13, torpedo, 8, 13, 43 
Mark 14 torpedo, 57, 120 
Mark 18 torpedo, 8, 18, 28, 120 
Mark 20 torpedo, 159 
Mark 26 torpedo, 20 
Torpedoes, control systems; see Air- 
craft launched torpedoes, con- 
trol systems, Antisubmarine 
torpedo control systems, Anti- 
surface-ship torpedo control sys- 
tems 

Torque due to torpedo propeller, 18 
Trajectory of torpedo, analysis, 41 
equations of motion, 37-45, 74 
Transducer, directivity index, defined, 
64 

Traducer assembly, stabilized, 147 
Transducer axis, defined, 64 
Transducer elements; laminated stack 
magnetostrictive, 76, 85, 140 
piezoelectric crystal, 121, 149 
Quartz sandwich type, 149 
Transducer mounting, British Bowler 
system, 154 

Transducers, echo-ranging; see also 
Hydrophones, torpedo 


British Dealer System, 104 
British Trumper System, 149 
BTL 157C System, 121 
GEN0181 system, 76, 86 
Geier system, 140 

general design considerations, 50-52, 
70 

ORL project 4 system, 106 
T ransmitter circuits , echo-ranging ; 
British Bowler system, 154 
British Trumper system, 149 
BTL 175C system, 124 
GE N0181 system, 76 
Geier 1 system, 141 
General design considerations, 160 
HUSL N0181 system, 91 
ORL project 4 system, 106 
Transmitter-receiver switching circuit, 
BTL 157C system, 125 
Trigger circuit, BTL 157C system, 
132 

Trumper system, British, 149-153 
Turning radius of torpedo, effect on 
homing range, 56 

Universal torpedo, proposed, 6 

Variable-speed torpedo, proposed, 6 
Vertical steering control, analysis, 
39 

British dealer system, 104 
ExFER 42 mine, 57 
GE N0181 system, 82 
general design considerations, 49, 
72-75 

HUSL N0181 system, 103 
restricted to 250 yd. range, 118 
Vertical steering relay, ORL project 4 
system, 115 
Vortex cavitation, 10 
Vortex pressures, 10 

Wake avoiding steering system, 121, 
130, 139, 144-148, 158 
Wake following torpedoes and mines, 
5, 57, 156 

Water conditions effecting homing tor- 
pedoes; effect of turbulent sur- 
face layer, 21 

effect of depth steering, 49 
water background noise, 7, 23, 25 

Zero spectrum sound level, def., 63 






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